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MEDTCAL  LIBRARY  COLUMBIA  UMVERSIS| 

COLLEGE  OP  PHYSICIANS  AND  SUROEOSa 

67.0  WET  163th  STREET 


- 


ON  AFTER-IMAGES 


SHEPHERD  IVORY  FRANZ,  A.B. 


SUBMITTED    IN    PARTIAL    FULFILMENT    OF    THE    REQUIREMENTS    FOR 

THE    DEGREE    OF    DOCTOR    OF    PHILOSOPHY 

IN    THE 

Faculty  of  Philosophy,   Columbia  University 


COLUMBIA  UNIVERSITY 

DEPARTMENT  OF  PHYSIOLOGY 

College  of  Physicians  and  Surgeons 
fst  w£8t  fifty  ninth. street 

NEW  YORK 


NEW  YORK 

JUNE,   1899 


ON   AFTER-IMAGES 


SHEPHERD  IVORY  FRANZ,  A.B. 


SUBMITTED    IN    PARTIAL    FULFILMENT    OF    THE    REQUIREMENTS    FOR 

THE    DEGREE    OF    DOCTOR    OF    PHILOSOPHY 

IN    THE 

Faculty  of  Philosophy,   Columbia  University 


NEW  YORK 

JUNE,   1899 


PRESS   OF 

The  New  Era  Printing  Company, 
Lancaster,  Pa. 


CONTENTS. 

Page. 

Introduction 2 

Part  I.     Experimental 3 

Sec.  1.  Apparatus  and  Methods 3 

Sec.  2.  Threshold 4 

Sec.  3 .   Latent  Period 11 

Sec.  4.  Duration 18 

Sec.  5.   Fluctuations 37 

Sec.  6.  Qualitative  Changes 38 

Sec.  7.  Space-relations 41 

Sec.  8.   Retinal  Transfer 44 

Part  II.     Historical  and  Descriptive 51 

Sec.  9.   Relation  to  Sensation,  to  Memory,  and  to  Imagination  51 

Sec.  10.   History 54 

Bibliography 60 


Digitized  by  the  Internet  Archive 

in  2010  with  funding  from 

Open  Knowledge  Commons  (for  the  Medical  Heritage  Library  project) 


http://www.archive.org/details/onafterimagesOOfran 


ON  AFTER-IMAGES. 
Introduction. 

After-images  were  first  described  in  the  De  Somniis  by 
Aristotle,  who  regarded  them  as  closely  allied  to  the  centrally 
excited  images  of  the  dream-state  ;  they  were  rediscovered  and 
described  anew  by  St.  Augustine,  and  again  by  the  Arab  Al- 
hazan  (a  student  of  Aristotle's  works)  in  the  eleventh  century. 
Peiresc,  in  describing  these  appearances  in  the  seventeenth 
century,  thought  that  he  had  discovered  a  new  phenomenon. 
Among  other  prominent  investigators  to  note  and  experiment 
with  the  images  were  Boyle,  Newton,  Buffon,  Goethe,  the  elder 
Darwin  and  Fechner. 

These  vestiges  of  sensation  owe  their  present  interest  in 
large  measure  to  their  seemingly  twofold  character,  being  al- 
lied both  to  sensation  and  to  memory-  and  imagination-images. 
To  the  earlier  observers  after-images  were  more  nearly  like 
the  images  of  the  imagination ;  later,  they  were  considered 
almost  a  sensation ;  whilst  most  recently  the  original  position  of 
Aristotle  is  again  prominent.  In  the  history  of  after-images  we 
seem  to  have  an  epitome  of  the  interrelations  of  physics,  physi- 
ology and  psychology ;  and  probably  no  other  single  phenome- 
non is  so  good  an  example  of  the  growth  of  experiment  and 
measurement  in  psychology. 

The  succeeding  portions  of  the  present  monograph,  apart 
from  the  bibliography,  deal  respectively  with  (i)  an  experi- 
mental analysis  of  the  conditions  affecting  the  production,  the 
duration,  the  latent  period,  the  space-relations,  etc.,  of  the  after- 
image, and  with  (2)  a  history  of  the  phenomena  and  their  rela- 
tion to  sensation,  to  imagination  and  to  memory. 

In  the  bibliography  I  have  given  references  to  what  seem 
the  most  important  contributions  on  the  general  subject  of  after- 
images.      In    Professor    Koenig's    excellent   bibliography1  of 

1Helmholtz,  Handbuch  der  Physiologischen  Optik,  2te  Aufl. 


2  5.   /.   FRANZ. 

vision  will  be  found  references  to  almost  all  the  work  on  this 
subject  done  previous  to  1894.  In  the  numbers  of  the  Psy- 
chological Index,  and  in  the  bibliographies  in  the  Zeitschrift 
filr  Psychologie  and  in  the  ZSAnnee  Psychologique  will  be 
found  references  to  current  literature  since  1893. 

The  experiments  to  be  subsequently  described  were  all 
made  in  the  psychological  laboratory  of  Columbia  University  ; 
and  as  subjects  eleven  advanced  students  in  psychology  took 
part,  viz.,  B,  C,  D,  F,  G,  H,  Ho,  K,  M,  S  and  W.  Other 
subjects,  too  numerous  to  mention  individually,  chosen  for  their 
naivete,  were  used  for  check-experiments.  To  all  I  am  greatly 
indebted  for  their  help  and  suggestions  throughout  the  progress 
of  the  research. 

The  terminology  used  in  the  succeeding  portions  is  as  fol- 
lows :  (1)  Positive  after-image  is  an  after-image  in  which  the 
image  and  its  background  bear  the  same  intensity-relation  as  in 
the  stimulus.  (2)  Negative  image  is  one  in  which  the  relation 
of  intensity  is  reversed.  Thus,  if  the  stimulus  is  a  red  cross  on 
a  black  background,  and  the  resultant  after-image  is  projected 
on  a  white  wall,  the  image  will  be  darker  than  the  background, 
and  accordingly  negative ;  if  the  image  is  projected  on  gray,  it 
may  be  darker  or  lighter  than  the  field  and  either  negative  or 
positive.  In  either  case  the  image  maybe  of  the  same  color  as 
the  original  or  of  a  different  color.  Four  different  kinds  of 
images  must  then  be  distinguished,  viz.  : 

same-colored.  ,T         .         C  same-colored. 


_     .  .         C  same-colored.  ,T       ,.         C 

Positive    <      ,  ,  Negative    < 

C  other-colored.  C 


other-colored. 


Wundt's  definitions,  (1)  that  the  image  is  positive  when  it  is 
of  the  same  or  greater  intensity  than  the  stimulus,  and  negative 
when  it  is  of  lesser  intensity  ;J  and  (2)  that  positive  images  are 
same-colored  and  negative,  are  other-colored,2  have  not  been 
accepted  by  the  most  recent  and  best  writers. 

1  Grundziige  der  Physiologische  Psychologie,  4  Ed.,  I.,  513. 

2  Human  and  Animal  Psychology,  Eng.  trans.,  p.  109. 


ON  AFTER-IMAGES.  3 

Part  I.     Experimental. 

Section  i.  Apparatus  and  Methods. — The  apparatus 
used  throughout  the  series  on  threshold,  latent  period  and  dura- 
tion was  that  used  by  Fullerton  and  Cattell,1  adapted  by  the 
writer  for  the  present  purposes.  The  accompanying  illustra- 
tion shows  the  instrument  from  the  standpoint  of  the  experi- 
menter. 


The  apparatus  consists  of  a  vertical  screen  (6")  with  an 
adjustable  opening  (Zf),  of  a  long  arm  (A)  and  of  a  screen 
pendulum  (P).  At  the  opening  (H)  arrangements  were  made 
for  inserting  ground-glass  plates  for  equalizing  the  illumination 
of  the  stimulus  and  for  rapidly  changing  the  areas.  The  long 
arm  (A )  carried  a  kerosene-lamp  (Z)  (in  the  experiments  upon 
duration  and  latent  period  an  electric  arc  lamp),  the  changes  in 
intensity  being  made  by  moving  the  source  of  light  toward  and 
from  the  observer.     On  the  pendulum  (.P)  was  a  screen,  which 

1  Perception  of  Small  Differences,  p.  135;  University  of  Pennsylvania  Press, 
1892. 


4  5.    I.    FRANZ. 

in  its  usual  position,  as  the  pendulum  is  held  up  by  the  electro- 
magnet (M),  shuts  off  the  light  from  the  area  to  be  illumined 
at  (If).  When  the  pendulum  is  released,  however,  the  light 
shines  on  the  area  for  one  second,  and,  returning,  it  is  caught 
up  by  the  electro-magnet.  The  key  (K)  was  used  to  make 
and  break  a  circuit  to  the  electro-magnet  (M)  for  releasing  and 
holding  the  pendulum.  Before  the  upright  screen  (S)  is  a 
tachistoscope  (not  shown  in  the  figure),  with  drop  screens  to 
give  a  light-stimulation  of  ToV<P  ihj>  or  iV  second-  The  whole 
instrument  was  placed  in  a  dark-room  before  a  cabinet  with  an 
opening  opposite  (H).  All  experiments,  unless  otherwise  noted 
hereafter,  were  made  in  a  dark-room.  The  subject  remained 
inside  the  dark-cabinet  for  from  ten  to  fifteen  minutes  for  adap- 
tation before  any  experiments  were  made.  Then,  with  the  eyes 
on  a  level  with  the  stimulating  light,  the  area  was  uncovered, 
covered  again,  and  the  subject  reacted  in  the  manner  appropri- 
ate to  the  series  of  experiments. 

Section  2.  The  Threshold.1 — Not  every  stimulus  is  fol- 
lowed by  a  sensation ;  a  sound  may  not  have  the  requisite  in- 
tensity ;  the  weight  be  not  sufficiently  heavy  :  the  light  not  large 
enough  or  long  enough  continued.  In  like  manner  minimal 
amounts  of  energy,  time  and  space  stimulation  will  fail  to  pro- 
duce an  after-image.  Some  questions  immediately  suggested 
are:  What  intensity  of  light  will  produce  the  after-sensation? 
How  large  an  area  is  necessary  to  get  an  after-image?  How 
long  must  the  stimulus  last  to  leave  its  effect?  Then,  we 
may  ask  whether  or  not  there  is  a  relation  between  the  varying 
changes  in  the  light,  whereby  a  small  area  may  be  counterbal- 
anced by  a  longer  stimulation  or  by  a  greater  intensity,  andvice 
versa. 

Time. — As  noted  above,  three  variables  were  used,  viz., 
time,  area  and  intensity.  With  a  fixed  area  and  intensity, 
what  time  must  the  light  stimulate  the  eye  to  produce  an  after- 
image? The  lamp  placed  at  25  cm.  from  the  screen  S,  giving 
an  intensity  of  -£$  c.p.,  and  the  area  64  sq.    mm.,  were  used 

1  Sec.  2  is  largely  reprinted  from  an  article  on  The  After-image  Thresh- 
old; Psychol.  Rev.,  II.,  130-136  (1896). 


ON  AFTER-IMAGES.  5 

as  constants  throughout  this  series.  The  time  was  varied  from 
ToV'tf  to  x  second.  Two  intermediate  steps  were  used,  y-J-g-  and 
-j'j-  second. 

Adaptation  completed,  the  subject  sat  in  position,  a  stimulus 
was  given,  and  the  subject  announced  the  appearance  or  non- 
appearance of  an  after-image.  Usually  thirty  to  forty  minutes 
were  taken  for  each  day's  experiments,  the  series  being  stopped 
before  fatigue  became  apparent.  For  each  subject  the  number 
of  experiments  upon  each  variation  in  time  was  one  hundred. 
Where  a  greater  or  less  number  were  made  the  small  numbers 
in  parentheses  in  the  appropriate  columns  of  the  tables  show 
how  many  tests  of  this  kind  were  made.  The  experiments 
were  all  made  with  the  eyes  open,  so  as  not  to  disturb  the  after- 
image. The  subject's  eyes  were  at  the  normal  distance  (about 
30  cm.)  from  the  stimulus  ;  his  head  was  steadied  by  a  support. 
Rests  were  taken  between  the  separate  experiments  to  allow  any 
trace  of  the  previous  image  to  disappear,  a  signal  was  given,  five 
seconds  were  allowed  for  preparation  and  the  light  was  shown. 

The  intensity  of  the  light  was  very  constant.  The  lamp  was 
trimmed  before  the  whole  series  of  experiments  ;  and  the  photo- 
metric determinations  made  before,  during  and  after  a  sitting 
showed  only  the  variations  likely  to  occur  in  any  series  with  the 
photometer  used  (Bunsen's).  A  fixation-point  could  not  be  se- 
cured throughout  the  experiments.  At  first  this  was  a  some- 
what disturbing  factor ;  but  as  the  light  used  as  a  stimulus  was 
so  small  and  the  after-image  so  indistinct,  it  was  judged  best 
not  to  have  a  fixation-point  of  light,  owing  to  the  confusion 
likely  to  result  from  mistaking  it  for  an  after-image.  With 
practice,  however,  the  observer  learned  to  look  for  the  stimulus 
in  the  proper  direction  ;  and  in  the  case  of  the  writer  in  not  over 
5  per  cent,  of  the  times  was  it  necessary  to  focus  the  eyes  con- 
sciously after  any  part  of  the  light  was  seen. 

The  following  table  shows  the  results  of  the  experiments  for 
time  obtained  from  two  observers,  C,  an  advanced  student  of 
psychology,  and  F,  the  writer,  upon  whom  the  experiments 
were  made  by  another  worker  familiar  with  the  apparatus. 
The  table  shows  on  the  first  lines  the  percentages  of  times  that 
after-images    appeared.     The    results    were    also    grouped   in 


5.    /.   FRANZ. 


series  of  tens,  and  the  variations  of  these  groups  were  calcu- 
lated. These  variations  are  shown  on  the  second  lines,  marked 
A.  var. 

Table  I. — Time. 


Time  in  sec. 
Subjects. 

I 

l 
TTT 

1 

1 
1000 

r      j  Per  cent. 
*■"      I  A.  var. 

■p,      j  Per  cent. 
r  "     1  A.  var. 

97 
3-5(7°) 

IOO 

95 
7- 

97 
4.2 

75 
17 

82.5 
8.3(120) 

12 
10.8 

19 
7-4 

Average. 

99 

96 

79 

r5-5 

As  noted  above,  100  experiments  of  each  kind  were  made,  ex- 
cept where  another  figure  is  shown  in  parentheses.  Thus,  for 
F,  with  the  stimulus  -j^-  second,  120  experiments  were  made; 
while  for  C,  using  one  second  stimulation,  only  70  trials  were 
made. 

The  accompanying  curve  shows  the  results  graphically — the 
abscissa  denoting  divisions  of  time,  the  ordinate  the  percentage 

of    times    after-images    ap- 
peared.    Only  the  points  for 

T0V0'  Tlhp  and  iV  seconds 
are  shown. 

The  shape  of  the  curve 
indicates  what  might  have 
been  expected  in  accord- 
ance with  the  results  of  ex- 
periments upon  other  time- 
phenomena  of  vision.  With 
the  shortest  time  after-im- 
ages seldom  appeared ;  then 
with  a  slight  increase  in  the 
duration  of  the  stimulus 
there  was  a  rapid  rise  in  the 
number  of  appearances,  fol- 
lowed by  only  a  slight  in- 
crease for  longer  stimulation.  The  same  character  of  curve  is 
found  in  investigating  the  time  it  takes  to  see  a  light  or  a  color. 


00- 

% 

jm^^j^^^^ 

80- 

r^^g^ 

60- 

1        TKscm®, 

<r0- 

1 

20- 

1         re«  2. 

sec. 


ON  AFTER-IMAGES.  7 

When  shown  for  a  very  short  time  the  color  can  never  be  seen ; 
but  as  the  time  is  slowly  increased,  it  becomes  very  plain,  but 
is  seen  no  oftener  and  becomes  no  plainer  if  the  time  be  further 
increased.  In  both  cases  (recognizing  a  color  and  seeing  after- 
images) the  stimulus  at  first  is  insufficient  to  make  the  sensation 
appear  above  the  threshold  of  conscious  experience.  This 
threshold  is  then  passed  suddenly,  and  the  stimulus  soon  reaches 
a  point  at  which  an  after-image  or  the  color  (as  the  case  may 
be)  is  always  seen,  unless  numerous  distractions  of  great  in- 
tensity interfere. 

This  effect  of  increase  in  the  time  of  stimulation  is  quite  in 
accord  with  the  commonly  accepted  theory  regarding  the  after- 
image :  that  the  phenomenon,  at  least  in  its  positive  form,  is 
due  to  inertia  of  the  retina.  From  this  point  of  view,  the  longer 
the  light  works  upon  the  retina — the  greater  the  amount  of 
energy  used — the  greater  will  be  the  effect  on  consciousness. 

Area. — A  corresponding,  though  not  a  proportionate,  in- 
crease in  the  number  of  appearances  was  noted  when  the  area 
was  varied  from  Jg  to  64  sq.  mm.  Six  areas  were  used — 
squares  of  J^,  y2,  1,  2,  4  and  8  mm.  on  each  side.  These  were 
made  on  ground  glass,  opaque  paper  being  glued  over  all  but 
the  small  area  wanted.  The  error  in  making  the  larger  sizes 
was  less  than  ^  mm.,  which  amount  is  inappreciable  in  com- 
parison with  the  whole  area.  Difficulty  was  experienced  in 
making  the  areas  under  1  mm. ;  but  from  numerous  ones  made 
the  best  were  chosen.  In  those  chosen  the  error  was  not  more 
than  J?  mm.  The  smallest  areas  were  measured  with  a  mag- 
nifying-glass,  and  those  were  chosen  in  which  the  errors  com- 
pensated each  other. 

The  intensity  of  the  light  was  kept  at  -^  c.  p.,  and  the 
time  of  stimulation  was  one  second.  With  these  constant,  an 
after-image  was  very  seldom  seen  when  the  smallest  area  was 
used,  while  the  largest  square  produced  an  image  at  all  times. 
The  same  precautions  that  were  observed  in  the  experiments 
upon  the  effect  of  time-changes  were  taken  in  this  series  and  in 
all  the  experiments  that  are  described  later. 

The  following  table  and  accompanying  curve  give  the  re- 
sults obtained.     The  figures  in  the  table  are  as  in  Table  I. : 


5.   /.   FRANZ. 


first  are  given  the  changes  in  stimulation,  then  the  percentage 
of  times  the  image  was  seen  under  these  conditions  and  then  the 
average  variations  of  the  experiments  when  grouped  in  sets  of 
ten.  The  abscissa  of-  the  curve  denotes  the  relative  amount  of 
the  stimulus ;  the  ordinate  shows  the  percentage  of  times  after- 
images were  seen. 

Table  II. — Area. 


Area  in  sq.  mm. 
Subjects. 

64 

16 

4 

1 

1 

1 
TB- 

M    /Percent. 
m-    I  A.  var. 

„      f  Per  cent, 
t  A.  var. 

100 

-(5o) 

100 

90 
7-5(8o) 

96 
4.8 

72 
14.8 

S8 
8.8 

52 
12.4 

57 
13 

27 
9.2 

31 
15-4 

20 
6 

8 
8 

Average. 

100 

93 

80 

54-5 

29 

*14 

CA. 


WRUSMOIL®. 


re«  D 


±±     I  +  sq.cm.  16 

Some  preliminary  work  in  which  a  different  standard  of  in- 
tensity was  used — 2V  candle  power — shows  the  same  effect  of 
area-increase.  These  experiments  were  made  before  the  gen- 
eral line  of  work  was  fully  planned.  Seventy  experiments 
were  made  with  each  area,  and  the  writer  was  the  only  subject. 
The  results,  because  of  their  confirmatory  character,  are  ap- 
pended : 

Table  III. — Area. 


Area  in  sq.  mm. 

64 

16 

4 

1 

1 

_.        f  Per  cent. 
r  ■     \  A.  var. 

96 

5 

89 
10 

67 
14.7 

41 
10 

19 
9-9 

ON  AFTER-IMAGES.  9 

A  frtori,  one  might  not  expect  such  an  increase  in  the  num- 
ber of  appearances  to  follow  an  increase  in  the  size  of  the 
stimulus.  For,  with  intensity  and  a  time  of  stimulation  constant, 
one  might  well  say  that  the  structure  of  the  eye  is  such  that 
only  the  number  of  stimulated  elements,  not  the  stimulation  of 
the  individual  elements,  changes  with  any  increase  in  size  of 
the  physical  stimulus.  This  is  undoubtedly  true,  and  the  result 
is  paralleled  by  the  results  of  experiments  upon  the  extensive 
threshold  of  vision.  A  color  or  a  light  cannot  be  seen  when 
only  a  small  area  is  present ;  but  on  increasing  the  size  it  is 
readily  perceived.  This  may  be  analogous  to  the  so-called 
'  summation  process,'  or  it  may  and  probably  can  be  resolved 
into  a  matter  of  the  attention.  As  a  matter  of  fact,  the  largest 
area  may  not  produce  any  more  after-images  than  the  smallest. 
The  apparent  difference  may  be  the  result  of  inability  of  the 
smallest  sensations  to  fix  the  attention. 

Intensity. — A  similar  series  was  made  changing  the  inten- 
sity of  illumination.  The  lamp  was  moved  along  the  arm  (A) 
of  the  apparatus  (see  Fig.  i)  so  as  to  give  intensities  varying 
from  23-  to  ^jj-jj  c.p.  Three  intermediate  steps  were  chosen 
for  convenience — £$,  ^-J^  and  -gj^  c.p.  The  amounts  of 
energy  in  these  experiments  had  an  effect  corresponding  to 
what  was  found  for  the  changes  in  the  time  and  in  the  area  of 
stimulation.  The  results  obtained  and  a  graphic  representation 
follow : 

Table  IV. — Intensity. 


Intens.  in  c.p. 
Subjects. 

TS 

1 

50 

1 

8~0"0~ 

1 

3200 

„      f  Per  cent. 
\A.   var. 

,-,     ("Percent. 
F>    \A.var. 

IOO 

—  (80) 

IOO 

94 

7 
96 

5-4 

43 

25  (no) 

44 
19-5(13°) 

17 
20.8(110) 

15-5 
13-5  (13°) 

3-6 

1 

1.8 

Average. 

IOO 

95 

46 

46 

i-5 

Two  results  in  Tables  I.,  II.,  III.  and  IV.  should  be  noted : 
(1)  the  close  correspondence  of  the  observers,  and  (2)  the 
comparatively  large  variation  for  the  different  individuals.  The 
first  of  these   observations   is,  I  believe,  of  little  significance. 


io  S.   I.   FRANZ. 

The  great  variation,  however,  seems  to  point  to  a  considerable 
mental  influence  affecting  the  appearances.  This  will  probably 
be  more  clearly  brought  out  in  the  discussion  of  the  experiments 
upon  the  influences  determining  the  duration  of  the  after-image. 

I00-] 

/o 


F!@MI4. 


-i-  _!_  _i-  c.p.  _n 

3200    800  200  r  so 

Summary. — Experiments  were  made  upon  four  trained  sub- 
jects to  find  the  amount  of  time-,  area-  and  intensity-stimula- 
tion that  would  produce  an  after-image  (conscious).  It  was 
found  that  to  see  an  after-image  75  per  cent,  of  the  times  the 
eye  is  stimulated,  it  is  necessary  to  have  as  stimulation  (1)  for 
yJ-q  second  and  64  sq.  mm.  surface  of  light,  an  intensity  of 
fa  c.  p. ;  or  (2)  for  one  second  exposure,  an  area  of  4  sq.  mm. 
and  an  intensity  of  fa  c.p.  ;  or  (3)  for  1  second  exposure,  an 
area  of  64  sq.  mm.  and  a  light  of  ^-^  c.  p. 

The  average  results  for  the  three  series  are  indicated  in  the 
following  curve,  which  gives  a  general  view  of  the  relative 
effects  of  the  physical  units.  It  will  be  seen  that  the  effects  of 
time,  intensity  and  extensity-changes  are  varied.  This  shows 
that  for  physiological  or  mental  processes  the  units  of  physical 
energy  do  not  counterbalance.  The  compensating  relation 
seems  to  be  a  rather  complex  one.  In  its  simplest  form  this  re- 
lation may  be  stated  as  follows  :  squaring  the  time  equals  doub- 
ling the  intensity  or  quadrupling  the  area ;  and  vice  versa,  re- 
ducing the  area  to  one  quarter  equals  halving  the  intensity  or 
taking  the  square  root  of  the  time. 


ON  AFTER-IMAGES.  II 

Based  upon  the  well-known  phenomena  of  color-mixing  by 
means  of  Maxwell's  discs,  the  objection  may  be  raised  to  these 
conclusions  that  lights  of  much  less  intensity  produce  after- 
images, inasmuch  as  colors  fuse  in  any  illumination.  While  this 
is  true,  it  should  not  be  forgotten,  on  the  other  hand,  that  the 
after-images  producing  the  fusion  are  not  consciously  perceived 
as  after-images.  The  usually  low  intensity  of  light  in  conjunc- 
tion with  a  short  time  of  stimulation  does  not  produce  an  after- 
image that  can  be  consciously  noted.  Moreover,  the  overlap- 
pings  of  images  and  succeeding  stimuli  produce  a  total  effect 
which  lacks  many  of  the  distinctive  features  of  the  simple  after- 
image. 

100 


T0Z&? 


TTOI 


1F0(§MKI 


During  the  three  series  of  over  3000  experiments  only  five 
times  were  negative  after-images  seen.  These  were  noticed 
only  with  the  largest  area,  the  longest  time  and  the  greatest  in- 
tensity, and  always  toward  the  close  of  an  hour's  session  after 
the  eyes  had  been  stimulated  forty  or  fifty  times.  This  fact 
would  seem  to  support  the  hypothesis  that  the  negative  after- 
image is  due  to  eye-exhaustion. 

Section  3.  The  Latent  Period. — That  a  light  does  not 
immediately  produce  an  after-image  is  now  well  known.  The 
observation  seems  to  have  been  made  first  by  Young,1  in  1872, 

iC.  A.  Young,  Note  on  Recurrent  Vision;  Taylor's  PJiilos.  Mag.,  4  Ser. 
XLI1L,  343-345>  1872. 


12  6'.    /.   FRANZ. 

who,  in  using  a  Holtz  machine  which  gave  a  large  spark,  noted 
that  if  a  single  spark  was  made  any  conspicuous  object  in  the 
darkened  room  was  seen  at  least  twice — with  an  interval  of  a 
trifle  less  than  a  quarter  of  a  second — the  first  time  vividly, 
the  second  time  faintly ;  often  the  object  was  seen  a  third  and 
sometimes  (but  only  with  great  difficulty)  even  a  fourth  time. 
This  interval  between  the  sensation  and  the  after-image  has 
been  called  the  latent  period. 

Young's  observation  seems  to  have  been  quite  forgotten  for 
several  years  till  the  appearance  of  Bidwell's  note  on  the  image 
following  an  illumined  vacuum  tube.1  Bidwell  found  that,  if  a 
Geissler  tube  be  revolved  at  the  rate  of  once  in  three  seconds, 
the  tube  is  followed  at  a  distance  of  about  forty  degrees  by  a 
ghost-like  image  of  the  original.  If  the  rotation  stopped,  the 
ghost  moved  on,  merging  finally  with  the  tube.  The  latent 
period  in  this  case  was  about  one-third  of  a  second.  The  ob- 
servation was  repeated  upon  about  ten  people,  with  approxi- 
mately the  same  results. 

Independent  observers 2  at  the  same  time  announced  varia- 
tions in  the  experiment,  and  the  suggestion  was  made  that  the 
intermittent  appearance  of  lightning  might  be  due  to  a  similar 
cause.  The  effect  may  be  produced  in  a  simple  manner  with 
a  match  or  a  piece  of  glowing  coal  revolved  by  hand.  The 
ghost-image  is  seen  to  follow  close  after  the  stimulus.  After 
some  experience  even  a  pencil  or  a  bright-colored  book  when 
moved  shows  the  phenomenon. 

Later  and  more  exact  measurements  of  the  appearances  have 
been  made  by  Bidwell,3  Charpentier4  and  Hess.5  Charpentier 
repeated  Bidwell's  observations  and  noted  a  second  appearance 
after  the  sensation,  as  Young  and  Bidwell  had.  This,  he  adds, 
is  easier  to  note  in  indirect  than  in  direct  vision.     A  similar  os- 

1  S.  Bidwell,  On  Certain  Spectral  Images  Produced  by  a  Rotating  Vacuum 
Tube;  Nature,  XXXII.,  30-31,  1885. 

2  Davis,  Laurin,  Newall  and  others. 

3  On  the  Recurrent  Images  following  Visual  Impressions;  Proc.  Roy.  Soc, 
LVI.,  132-145,  1894. 

4  Reaction  oscillatoire  de  la  retine  sous  Tinnuence  des  excitations  lumineuses  ; 
Arch,  de  Physiol.  (Ser.  5),  IV.,  54I-5S3.  l892- 

6Untersuchungen  liber  die  nach  kurzdauernder  Reizung  des  Sehorgans; 
Archivf.  die  gesammte  Physiol.,  XLIX.,  190-208,  1891. 


ON  AFTER-IMAGES.  13 

dilatory  effect  was"  found  by  Charpentier  in  using  a  black  disc 
with  a  comparatively  small  white  sector.  Revolving  the  disc 
once  every  second  or  every  two  seconds,  and  fixating  the  center, 
the  white  sector  was  seen  to  be  crossed  in  places  by  dark  bands. 
These  seem  closely  related  to  the  disappearances  and  the  reap- 
pearances of  the  after-image,  but  need  more  careful  study  in  the 
light  of  Exner's  work  upon  the  progress  of  a  visual  sensation. 

Bidwell's  later  work  is  largely  a  repetition  of  his  earlier  ob- 
servations, but  using  spectrum  colors  instead  of  the  illuminated 
Geissler  tube.  A  spectrum,  revolving  every  1  1/2  seconds,  was 
thrown  on  a  screen  and  the  ghost-image  observed.  The  ghost 
followed  the  spectrum  at  a  distance  of  about  50  degrees.  The 
latent  time  was  0.2  second.  It  is  interesting  to  note  that  Bidwell 
observed  no  image  following  red  or  violet  light.  The  recurrent 
image  of  an  illumined  slit  in  an  opaque  disc  was  found  to  follow 
after  0.15  second.  Another  disc,  15  cm.  diameter,  having  two 
opposite  radial  slits  1/2  mm.  wide,  was  then  rotated  one  turn  a 
second  before  illumined  ground  glass.  As  observed  from  a 
distance  of  1  1/2  meters,  each  slit  seemed  to  give  four  and  pos- 
sibly five  luminous  images  arranged  like  the  ribs  of  a  fan.  The 
usual  blue  recurrent  image  could  also  be  seen  following  the 
images  of  the  radial  slits  at  an  angle  of  80  degrees.  Through- 
out, the  eye  was  fixed  upon  the  center  of  the  disc.  This  obser- 
vation is  readily  confirmed,  but  seems  not  to  be  understood.  The 
fact  that  Bidwell  speaks  of  the  oscillations  and  the  ghost  sepa- 
rately seems  to  indicate  that  he  did  not  believe  they  were  similar 
in  character.  Even  if  we  admit  Charpentier's  contention  of  an 
oscillatory  process  in  the  retina,  which  makes  a  white  disc  at 
any  phase  seem  alternately  dark  and  light,  we  could  not  admit 
it  for  the  dark  opaque  disc  of  Bidwell  nor  for  the  after-image 
accompanying  it. 

One  fact  seems  to  have  escaped  notice — at  least  it  has  not 
been  recorded.  In  the  simple  experiment  with  a  burning  coal 
or  with  a  Geissler  tube  the  ghost  follows  the  stimulus  ;  but,  in 
addition,  there  is  an  after-image  of  the  path  taken  by  the  light. 
In  this  image  the  oscillations  described  by  Bidwell  do  not  occur. 
None  of  the  fluctuations  usually  apparent  in  the  after-image  are 
noted  in  this  case,  the  image  gradually  fading  away. 


5.    /.    FRANZ. 


Experiments  of  a  different  character  have  been  made  by 
Hess.  In  his  work  moving  objects  were  not  used,  momentary 
illumination  of  a  stationary  stimulus  taking  the  place  of  the  re- 
volving discs.  A  photographic  shutter  gave  exposures  of  y^-g- 
to  2T5-  second.  Hess  discovered  a  negative  after-image  of 
very  short  duration  following  immediately  upon  the  termination 
of  the  stimulus.  This  preceded  what  is  commonly  known  as 
the  positive  after-image. 

The  nature  of  the  experiments  in  the  present  work  is  more 
nearly  related  to  the  work  of  Hess  than  to  that  of  Charpentier 
and  Bidwell.  A  stationary  light  was  used  and  the  reaction-time 
taken.  As  in  the  work  on  threshold,  I  endeavored  to  find  the 
relative  effects  of  varying  stimuli  in  the  three  respects  of  in- 
tensity, time  and  area.  The  apparatus  used  was  essentially 
the  same  as  that  described  in  Section  1.  The  kerosene  lamp, 
however,  was  replaced  by  an  arc  light  of  about  1000  c.  p. 
The  intensities,  as  given  to  the  subjects,  were  1,  Jg-  and  j-^-g-  c. 
p. ;  these  were  obtained  as  in  the  threshold-experiments.     \,  1 

and  4  sq.  cm.  in  squares  were 
(LATIEMT  IPH^O©®  used  as  areas,  and  the  varying 
times  were  yjj-g-,  yg-  and  1  sec- 
ond. As  constants  were  used 
yT  c.  p.,  1  sq.  cm.  and  1  sec. 
Only  one  of  the  factors  was 
varied  at  any  one  time.  A 
chronoscope  was  placed  in 
series  with  a  reaction  key  and 
the  pendulum  (P)  and  the  drop 
tachistoscope.  The  observer 
was  instructed  to  react  the  in- 
stant the  after-image  appeared, 
and  the  time  between  the  end 
of  the  stimulus  and  the  begin- 
ning of  the  after-image  could 
thus  be  read  direct  from  the 
chronoscope. 

The  individual  variation  is  quite  large,  owing  to  the  mental 
attitude  of  the  subject,  one  reacting  to  the  first  approximation 


100  10 


sec. 


ON  AFTER-IMAGES. 


15 


of  an  after-image,  another  waiting  till  he  was  certain  that  the 
appearance  was  what  he  sought.  Although  his  attitude  was 
constant  with  each  individual,  we  shall  not  be  able  to  compare 
the  subjects,  and  to  group  them  for  an  average  representing 
a  typical  result.  So  also  the  individual  attitude  toward  the 
stimuli  made  other  variations  in  the  results.  To  one  the  stimuli 
of  greatest  intensity  were  too  bright ;  at  times  another  could 
scarcely  get  an  after-image  with  the  smallest  area. 

Accompanying  will  be  found  the  tables  and  curves  for  the  av- 
erage results  for  time-,  area-  and  intensity-experiments  upon  four 
subjects.  Twenty  experiments  were  made  upon  each  variable. 
In  the  tables  the  average  time  between  the  stimulus  and  the 
first  appearance  of  the  after-image  is  first  given,  and  this  is  fol- 
lowed by  the  average  variations  of  the  twenty  experiments. 

Table  V. 

Time  :  constants,  Tlj  candle  power,  1  sq.  cm.- 


Stimulus  in  Seconds. 
Subjects. 

.01 
Aver.           A.  Var. 

.1 
Aver.           A.  Var. 

1.0 

Aver.          A.  Var. 

B. 
D. 
G. 
W. 

I.62 
•49 
•76 
■7i 

.42 
.19 
.22 
•13 

.99                  .12 

•83              -35 
.61               .23 

.46          .14 

I.76 
•65 
•71 
.92 

•74 
.11 
.18 
.20 

Aver. 

90 

72 

1. 01 

?00 

\.                                HATIOT  PUBOO®. 

sec. 

\     ^^^                                 "** 

1.75- 

\ 

1.50- 

\                                                      " — 

|_ 

125- 

\                mm. 

L00 

\ 

M*> 

•75- 

\                                                 <s 

•50- 

© 

-25- 

F0669R1  7, 

sei.cm. 


1 6  S.   I.   FRANZ. 

Table  VI. 

Area:  constants,  I  sec,  and  j^  c.  p. 


Area  in  sq.  cm. 

X 

: 

\ 

Subjects. 

Aver. 

A 

Var. 

Aver. 

A.  Var. 

Aver. 

A.  Var. 

B. 

2. II 

■65 

1.76 

•74 

1.50 

.46 

D. 

I.91 

) 

.2,5 

■65 

.11 

•58 

.07 

G. 

•63 

■13 

•71 

.18 

■65 

.11 

W. 

.90 

■15 

.92 

.20 

.86 

■13 

Aver. 

i-39 

1. 01 

.90 

Table  VII. 

Intensity  :  constants,  1  sec,  and  1  sq.  cm. 


Intensity  in  c.  p. 

.01 

[ 

I 

Subjects. 

Aver. 

A.  Var. 

Aver. 

A.  Var. 

Aver. 

A.  Var. 

B. 

i-39 

•34 

I.76 

•74 

3-36 

1.58 

D. 

•57 

.08 

•65 

.11 

■Si 

.14 

G. 

.67 

.12 

•71 

.18 

1.09 

•33 

W. 

•77 

•13 

.92 

.20 

.72 

■" 

Aver. 

■85 

I. OI 

1.42 

sec 

2.50' 


Summary . — While  the  grouping  and    averaging    of   these 
diverse  figures  cannot  be  well  justified,  I  have  done  so  because, 


OJST  A  FTER-IMA  GES.  1 7 

in  a  general  way,  the  figures  of  the  averages  seem  to  show 
what  ordinarily  would  be  expected.  With  the  lesser  times  the 
eyes  have  not  time  to  move  and  to  get  a  sensation  on  many  con- 
tiguous portions  of  the  retina.  Accordingly  when  the  time  of 
stimulation  is  short  the  after-images  are  clearer  and  more  easily 
recognized.  The  larger  areas  have  the  advantage  of  giving 
after-images  of  considerable  size,  which  attract  the  attention 
sooner  than  the  smaller  ones ;  while  the  great  intensities  seem 
to  be  so  blinding  in  their  effect  that  the  image  is  very  hard  to 
recognize.  I  append  a  curve  showing  the  comparative  effects 
of  the  variables,  but  with  full  knowledge  that  its  meaning 
should  not  be  taken  as  absolute,  but  only  as  an  indication  of  the 
results  that  a  much  larger  number  of  individuals  may  approxi- 
mate. 


ISO 

sec 

1.25 
1.0& 
.75  ■ 

.50 
.25 


FDStilRE  9. 


In  the  existing  theories  explaining  the  phenomena  no  reason 
is  noted  for  any  variation  in  this  latent  period.  It  seems  prob- 
able that  an  explanation  must  be  looked  for  in  the  mental  at- 
titude of  the  subject.  Such  an  explanation  would  not  only  ac- 
count for  the  great  individual  differences,  but  would  also 
account  for  the  great  variation  of  the  same  individual  under 
precisely  the  same  physical  conditions.  The  subject  G,  who  is 
distinctly  visual,  accustomed  to  note  variations  in  color,  etc., 
apparently  recognized  the  image  sooner  than  any  of  the  others 
who  were  not  so  trained.  This  mental  attitude  will  also  ex- 
plain why  the  after-image  of  the  largest  areas  was  seen  sooner 
than  that  of  the  smaller  one.  The  attention  in  this  case,  was 
probably  attracted  sooner  to  the  large  object.  The  conditions 
attending  the  changes  in  latent  time  for  intensity-  and  time-ex- 


1 8  S.   /.   FRANZ. 

periments  are  not  so  evident.  In  the  intensity-experiments  the 
subjects  B  and  G,  for  whom  the  i  c.  p.  stimulus  was  too  strong, 
dominated  thereby  the  totals.  Considered  in  connection  with 
the  results  of  the  experiments  upon  duration  (Section  4),  it 
seems  likely  that  the  attention  is  a  predominant  factor  in  all 
measurements  of  the  phenomena. 

This  view  is  further  strengthened  if  the  average  variations 
under  the  different  conditions  are  considered.  Usually  those 
conditions  which  the  subject  considered  favorable  show  the 
least  variation.  Thus,  B  felt  that  the  Tl_-second  exposure  was 
too  short  a  time  for  him  to  see  the  after-image,  and  the  great 
variation  shows  that  there  must  have  been  some  great  disturb- 
ing factors.  The  same  occurred  with  both  G  and  B  under 
stimulation  of  I.  c.  p.,  which  was  considered  by  both  as  too 
strong. 

Section  4.  Duration. — The  earliest  accounts  of  the  du- 
ration of  after-images  seem  to  be  those  given  by  Boyle  and  by 
Newton.  In  his  work  on  color,  Boyle  relates  that  a  man  who 
had  looked  through  a  telescope  at  the  sun  without  a  protecting 
medium  had  his  eyesight  so  injured  that  thereafter  whenever  he 
looked  toward  windows  or  other  bright  objects  he  fancied  he 
saw  a  globe  of  light  about  the  size  of  the  sun.  This  appearance 
was  a  source  of  trouble  to  the  man  for  nine  or  ten  years. 

To  Locke,  who  had  called  his  attention  to  this  interesting 
observation,  Newton  wrote  an  account  of  a  similar  personal  ex- 
perience. The  letter,  which  contains  further  observations  of 
interest,  is  quoted  fully  i1 

"  The  observations  you  mention  in  Mr.  Boyle's  book  of  colours,  I  once  made 
upon  myself  with  the  hazard  of  my  eyes.  The  manner  was  this  :  I  looked  a 
very  little  while  upon  the  sun  in  a  looking-glass  with  my  right  eye,  and  then 
turned  my  eyes  into  a  dark  corner  of  my  chamber  to  observe  the  impression 
made,  and  the  circles  of  colours  which  encompassed  it,  and  how  they  decayed  by 
degrees,  and  at  last  vanished.  This  I  repeated  a  second  and  a  third  time.  At 
the  third  time,  when  the  phantasm  of  light  and  colours  about  it  were  almost 
vanished,  intending  my  fancy  upon  them  to  see  their  last  appearances,  I  found, 
to  my  amazement,  that  they  began  to  return,  and  by  little  and  little  to  become 
as   lively  and  vivid  as  when  I  had  newly  looked  upon  the  sun.     But  when  I 

lD.  Brewster,  Memoirs  of  the  Life  of  Sir  I.  Newton;  Edinburgh,  1855, 
Vol.  I.,  p.  236  : 


ON  AFTER-IMAGES.  19 

ceased  to  intent  my  fancy  upon  them  they  began  to  vanish  again.  After  this 
I  found  that  as  often  as  I  went  into  the  dark  and  intended  my  fancy  upon  them, 
as  when  a  man  looks  earnestly  to  see  anything  which  is  difficult  to  be  seen,  I 
could  make  the  phantasm  return  without  looking  any  more  upon  the  sun  ;  and 
the  oftener  I  made  it  return  the  more  easily  I  could  make  it  return  again.  And 
at  length,  by  repeating  this  without  looking  any  more  at  the  sun,  I  made  such 
an  impression  upon  my  eyes  that,  if  I  looked  upon  the  clouds,  or  a  book,  or  any 
light  object,  I  saw  upon  it  a  round  bright  spot  of  light  like  the  sun,  and,  which 
is  still  stranger,  though  I  looked  tipon  the  sun  -with  my  right  eye  only,  and  not 
-ivilh  my  left,  yet  my  fancy  began  to  make  an  impression  upon  my  left  eye  as  well 
as  upon  my  rig/it.  For  if  I  shut  my  right  eye,  or  looked  upon  a  book  or  the 
clouds  with  my  left  eye,  I  could  see  the  spectrum  of  the  sun  almost  as  plain  as 
■with  my  right  eye,  if  I  did  but  intend  my  fancy  a  little  while  upon  it ;  for  at  first 
if  I  shut  my  right  eye  and  looked  with  my  left,  the  spectrum  of  the  sun  did 
not  appear  till  I  intended  my  fancy  upon  it ;  but  by  repeating  this  appeared  every 
time  more  easy.  And  now  in  a  few  hours'  time  I  had  brought  my  eyes  to  such 
a  pass  that  I  could  look  upon  no  bright  object  with  either  eye,  but  I  saw  the  sun 
before  me,  so  that  I  durst  neither  write  nor  read ;  but  to  recover  the  use  of  my 
eyes  shut  myself  up  in  my  chamber  made  dark  for  three  days  together,  and  used 
all  means  to  divert  my  imagination  from  the  sun.  For  if  I  thought  upon  him 
I  presently  saw  his  picture,  though  I  was  in  the  dark.  But  by  keeping  in  the 
dark,  and  employing  my  mind  about  other  things,  I  began  in  three  or  four  days 
to  have  some  use  of  my  eyes  again  ;  and  by  forbearing  to  look  upon  bright  objects 
recovered  them  pretty  well,  though  not  so  well,  but  that,  for  some  months  after, 
the  spectrum  of  the  sun  began  to  return  as  often  as  I  began  to  meditate  upon 
the  phenomena,  even  though  I  lay  in  bed  at  midnight  with  my  curtains  drawn." 

Several  interesting  observations  are  here  noted.  The  one  of 
most  importance  seems  to  be  that  regarding  the  effect  of  imagi- 
nation and  attention  upon  the  after-image.  This  will  be  dis- 
cussed later ;  as  will  also  the  one  regarding  the  transfer  of  the 
image  from  the  right  to  the  left  eye. 

The  philosopher  Tetens,  who  seems  in  some  way  to  have 
anticipated  the  modern  idea  of  mental  measurement,  mentions 
that  he  had  investigated  the  duration  of  the  after-image.  Only 
a  few  words  of  description  are  given,  and  details  were  left  for  a 
future  paper,  no  record  of  which  I  have  been  able  to  find. 

Some  few  records  of  the  duration  are  to  be  found  in  Fech- 
ner's  observation  of  the  color-changes,  but  the  material  is  so 
slight  as  to  be  of  little  value. 

Helmholtz  notes  that  the  duration  and  intensity  of  the  stimu- 
lating light  affect  the  duration  and  character  of  the  resultant 
after-image:  "The  greater  the  intensity  of  the  primary  light 
the  brighter  is  the  positive  after-image  and  the  longer  it  con- 


20  S.   I.   FRANZ. 

tinues."1     "  Greater  intensity  of   the  primary  light    gives   the 
negative  after-image  greater  clearness  and  duration."2 

The  most  extended  observation  of  the  changes  occurring  in 
connection  with  the  duration  of  the  phenomena  seem  to  be  those 
by  Titchener.3  Used  primarily  for  another  purpose  (see  Sec- 
tion 8),  the  observations  are  of  considerable  value  in  this  con- 
nection, because  they  were  made  upon  subjects  na'ive  as  to  the 
present  problem.  Dr.  Titchener  asked  his  subjects  to  record 
the  duration  of  various  after-images  in  a  stimulated  eye. 
These  experiments,  though  few  in  number  for  the  single  ob- 
servers, show  in  general  the  same  effect  as  the  more  extended 
series  by  nryself .  The  conditions  were  as  follows  :  The  sub- 
ject was  placed  before  a  screen  with  a  variable  sized  opening, 
which  was  to  be  illumined  by  a  lamp.  The  photometric  determi- 
nation of  the  light  is  not  given.  The  area-  and  time-stimulations 
were  changed  and  the  records  taken.  The  light  used  was 
made  vari-colored  by  the  use  of  gelatin  plates  ;  white  light  was 
not  used.  The  following  table  of  results  with  red  lights  will 
illustrate  the  general  tendency,  and  will  act  as  a  basis  for  com- 
parison with  the  present  work.  In  this  table,  which  I  have  con- 
structed from  the  individual  records  as  given  by  Titchener,  will 
be  found  the  averages  of  at  least  three  observers  for  each  series. 
The  maximum  number  of  experiments  made  under  any  con- 

Table  VIII. 


Area 

of  Stimulation  in  mm. 

0 

3° 

40 

5° 

o 

o 
w 

S 

9.9  (14) 

27.2  (9) 

g 

10 

12.5  (14) 

17.4(16) 

26.7  (8) 

0 

H 

15 

16.3  (15) 

22.     (14) 

29.7  (7) 

P 

§ 

20 

19.1  (10) 

23-9  (15) 

39-    (8) 

H 

25 

21.9  (11) 

30.1  (10) 

30.S  (7) 

1  Handbu 

2  Ibid.,  p 
3E.B.  Ti 

:h  der  Physiol.  C 

5°5- 
chener,  Ueber  b 

)ptik,  p.  503. 
noculare  Wirkui 

lgen  monoculare 

r  Reize ;  Philos. 

Stud.,  VIII.,  231-310  (1892). 


ON  AFTER-IMAGES.  31 

dition  by  a  subject  was  six,  the  minimum  two.  In  paren- 
theses will  be  found  the  number  of  experiments  used  in  making 
the  averages.  The  other  figures  give,  in  seconds,  the  average 
total  duration  of  the  after-image. 

The  individual  variations  from  these  averages  are  com- 
paratively large,  although  each  individual's  results  are  quite 
constant.  The  variability  is  about  the  same  as  was  found  in 
my  experiments  upon  the  effect  of  time-,  area-  and  intensity- 
changes. 

The  present  series  took  its  rise,  however,  largely  from  two 
incomplete  minor  studies  made  at  the  Columbia  Psychological 
Laboratory,  1891-93.  These  investigations  were  begun  by  Mr. 
L.  V.  Southack  and  Miss  E.  G.  Seebring,  respectively  ;  but  the 
results  and  methods  were  unknown  to  the  writer  till  after  the 
present  series  was  begun.  The  results,  which  have  remained 
unpublished,  I  give  in  the  appropriate  place,  with  the  corre- 
sponding present  series.  The  method  employed  by  both  ex- 
perimenters was  to  use  an  incandescent  lamp,  enclosed  in  a 
long  box,  to  illumine  a  ground-glass  Greek  cross  in  the  front  of 
the  box.  Each  arm  of  the  cross  was  one  square  centimeter. 
Variations  in  intensity  were  obtained  by  changing  the  position 
of  the  lamp,  and  variations  in  time  of  stimulation  were  meas- 
ured by  means  of  a  metronome.  The  results  were  written 
down  by  the  observers  after  each  experiment,  the  various  fluc- 
tuations being  noted  as  well  as  possible. 

The  apparatus  for  the  present  series  was  about  the  same  as 
that  used  in  the  work  on  latent  period ;  but  for  the  chronoscope 
a  kymograph  was  substituted.  Upon  this  a  continuous  record 
of  the  after-image  could  be  recorded.  The  time  of  stimulation 
was  noted  upon  the  drum,  and  the  subject  was  instructed  to  close 
the  key-circuit  as  soon  as  an  after-image  appeared,  to  keep  it 
closed  while  the  image  lasted,  and  to  break  and  make  the  circuit 
as  quickly  as  possible  when  any  changes  in  the  character  of  the 
after-image  took  place.  When  any  change  was  noted  by  the 
breaking  or  the  closing  of  the  circuit  the  subject  told  the  experi- 
menter its  character,  and  this  was  indicated  upon  the  kymo- 
graph-record. In  this  way  not  only  the  total,  but  the  actual 
time  of  duration,  the  fluctuations  and,  to  some  extent,  the  latent 


2  2  S.   I.   FRANZ. 

period  could  be  determined.  In  this  series  the  observations 
of  the  latent  period  were  open  to  criticism  owing  to  a  slight 
degree  of  inaccuracy  in  recording  the  end  of  the  stimulating 
light.  Such  an  error  was  relatively  large  for  the  short  times  of 
the  latent  period,  but  was  inappreciable  for  the  comparatively 
long  durations.  As  in  the  preceding  work  on  threshold  and 
latent  period,  the  three  physical  units  of  energy  were  varied. 
A  standard  stimulus  of  i  cm.,  J^  c.p.  and  5  seconds  was  chosen ; 
and  the  variables  were  \,  1  and  4  sq.  cm.,  y-g-^-,  yjr  and  1  c.p., 
and  1,  5  and  10  seconds.  It  was  found  impossible  to  record  the 
exact  time  of  each  fluctuation.  The  changes  in  the  character 
of  the  image  are  not  clear-cut,  but  there  is  a  fading  of  one  color 
into  another.  This  could  not  be  recorded  as  a  distinctive  color- 
change.  Even  the  usually  marked  changes  from  the  positive 
to  the  negative  phases  were  not  always  clear-cut. 

The  following  tables  and  curves  give  two  results  :  the  total 
time  of  the  after-image — i.  e.,  the  time  from  the  end  of  the 
stimulus  to  the  final  disappearance  of  the  after-image  ;  and  the 
actual  time — i.  e.,  the  total  time  minus  the  period  when  unseen. 
In  the  curves  the  total  durations  are  denoted  by  the  continuous 
lines,  and  the  actual  times  by  the  broken  lines. 

Time. — The  constants  in  these  experiments  were  an  intensity 
of  -jJ-jj-  c.p.  and  an  area  of  1  sq.  cm. 


Table  IX. 
Total  Duration  of  After-image. 


Time  of  Stimu- 
lation iu  seconds. 
Subjects. 

1 
Aver.  A.  Var.  No.  Exp 

5 
Aver.  A.  Var.  No.  Exp. 

10 

Aver.  A.  Var.  No.  Exp. 

K. 
H. 
F. 
M. 

32 

48 
29 
So 

6 

10 
4 
13 

20 
10 
10 
10 

56 

59 
32 

77 

24 
IS 

s 
29 

20 
10 

10 

10 

56 
§4 

34 
96 

14 

27 

4 
30 

20 
10 
10 
10 

Average. 

39-8 

50 

56 

So 

67-5 

SO 

ON  AFTER-IMAGES. 


23 


Table  X. 
Actual  Duration  of  After-image. 


Time  of  Stimu- 

5 

lation  in  seconds. 

Subjects. 

Aver.  A.  Var.  No.  Exp. 

Aver.  A.  Var.  No.  Exp. 

Aver.  A.  Var.  No.  Exp. 

K. 

26 

4 

20 

40 

12 

20 

40 

9 

20 

H. 

32 

9 

10 

38 

17 

10 

64 

23 

10 

F. 

25 

2 

10 

30 

5 

10 

29 

3 

10 

M. 

32 

10 

10 

54 

20 

10 

77 

20 

10 

Average. 

28.7 

50 

4°-5 

5° 

52-5 

5° 

DURATION 
lOOi 

sec 

80 
60 
40 


20 


DURATION 
100 

sec 

80 


AK1A., 


$mmE  00. 

"£      1  sq.cm.       ? 


1       s  sec.  10 

The  corresponding  series  in  Miss   Seebring's  experiments 
show  the  following  results  for  the  total  time  of  duration : 

Table  XI. 


Time  of  stimulation. 

3 

Aver.  A  Var.  No.  Exp. 

15 
Aver.  A.  Var.  No.  Exp. 

60 
Aver.   A.  Var.  No.  Exp. 

First  series. 
Second  series. 

21 
23 

9 

4 

5 
5 

47    1     6      I     5      j    i°7    1     5     I     S 
43    1     6      1     5      1    101    |     7     |     5 

Averages. 

22 

10 

45                   10 

104                        10 

Full  records  of   the  fluctuations  were  taken  only  for   the 
second  series,  and  the  actual  time  that  the  image  was  seen  can 


24 


5.   /.   FRANZ. 


be    given  only  for   these   experiments.     The   following    table 
shows  the  relation  between  the  total  and  the  actual  duration : 

Table  XII. 


Time  of  stimulation. 

Total  time. 
Aver.            A.  Var. 

Actual  time. 
Aver.            A.  Var. 

No.  Exper. 

3  sec. 
15  sec. 
60  sec. 

22.6 
42.6 
IOI.2 

A- 
6. 
7- 

10.9                 3.9 
36.                    4-4 
98.6                 8.4 

s 
s 

5 

In  Titchener's  results  the  same  effect  is  noted.  The  varied 
intensities  and  areas  do  not  permit,  however,  of  a  direct  com- 
parison of  the  three  series.  But  they  have  enough  in  common 
to  show  the  rapid,  but  not  proportionate,  increase  in  duration 
due  to  the  longer  times  of  stimulation. 

Intensity. — With  1  sq.  cm.  and  5  seconds  exposure  as  con- 
stants. 

Table  XIII. 
Total  Duration  of  After-image. 


Intensity  in  c.p. 
Subjects. 

Aver.   A 

.01 

Var.  No.  Exp. 

Aver. 

.1 
A.  Var. 

No.  Exp. 

Aver. 

1.0 
A.  Var.  No.  Exp. 

K. 
H. 

F. 

40 
29 
26     1 

12 

t 

20 
10 
IO 

56 
59 
32 

|      24 

5 

20 
IO 
10 

70 
84 
50 

1      * 
19 

3 

20 
10 
10 

Average. 

3i-8  | 

40 

49     J 

40 

68    1 

40 

Table  XIV. 

Actual  Duration  of  After-image. 


Intensity  in  c.  p. 
Subjects. 

Aver. 

.01 
A.  Var 

No.  Exp. 

Aver. 

.1 
A.  Var. 

No.  Exp. 

Aver. 

1.0 
A.  Var.  No.  Exp. 

K. 
H. 

F. 

28 
22 
25 

6 

7 
3 

20 

1       IO 

1       IO 

40 
38 
3° 

12 
17 

s 

20 
IO 
10 

52 
63 
50 

9 
18 

3 

20 
10 
IO 

Average. 

25 

|     4° 

36 

40 

55 

40 

Using  a  cross  with  1  sq.  cm.  arms  and  15  seconds  exposure, 
Mr.  Southack  found  for  himself  and  a  second  observer  the 
following  results.     The  standard  intensity,  1,  was  a  100  c.  p. 


ON  AFTER-IMAGES. 


25 


incandescent  lamp  at  a  distance  of  30  cm.  from  the  ground- 
glass  cross.  The  varying  intensities  were  obtained  in  the 
usual  manner. 

Table  XV. 

Total  Duration  of  After-image. 


Relative  Intensi- 
ties. 

Subjects. 

Av.  A-  Var. 

No. 
Exp. 

1 
Av.  A.  Var. 

No. 
Exp. 

1 
T 

Av.  A.  Var.1 

No. 

Exp. 

I 

No. 
Av.  A.  Var.1  Exp. 

A. 
B. 

3    1  5 
11.8I  1.4 

20 
IO 

10.6     1.5 
14.4  J    3.2 

20 
10 

II. 2 

18.  I 

i-7 
1.9 

20 
10 

60.7  1  10.7 
26.4 1    I 

20 
IO 

Area:  constants,  5  sec.  and  i  c.  p. 


Table  XVI. 

Total  Duration  of  After-image. 


Area  of  Stimulus 
in  sq.  cm. 

1 
¥ 

1 

4 

Subjects. 

Aver.  A.  Var.  No.  Exp. 

Aver.  A.  Var.  No.  Exp. 

Aver.  A 

.  Var.  No.  Exp. 

K. 

34 

15 

20 

56 

24 

20 

74 

24 

20 

H. 

3i 

12 

10 

59 

15 

10 

68 

17 

10 

Ho. 

24 

3 

5 

24 

4 

5 

45 

15 

5 

F. 

24 

S 

10 

32 

5 

10 

44 

6 

10 

Average. 

28 

45 

42.8 

45 

57-8 

45 

Table  XVII. 

Actual  Duration  of  After-image. 


Area  of  stimu- 

1 

4 

lus  in  sq  cm. 

T 

Subjects. 

Aver.  A.  Var.   No.  Exp. 

Aver. 

A.  Var. 

Vo.  Exp. 

Aver.   A.  Var.  N  o.  Exp. 

K. 

23 

7 

20 

42 

12 

20 

58 

10 

20 

H. 

21 

10 

10 

38 

17 

10 

45 

9 

10 

Ho. 

21 

3 

5 

20 

5 

5 

37 

11 

5 

F. 

23 

5 

10 

3° 

5 

10 

37 

11 

10 

Average. 

22 

45 

32 

45 

45 

45 

The  increase  in  duration  for  the  greater  intensities  and  the 
longer  times  of  stimulation  is  easily  explained  on  purely  physio- 
logical grounds.    In  both  cases  more  of  the  photo-chemical  matter 

1  The  average  variation  of  the  averages  when  the  experiments  were  grouped 
in  sets  of  five.  This  was  the  only  calculation  left  by  Mr.  Southack.  The 
records  of  the  individual  experiments  could  not  be  found. 


26 


5.    /.   FRANZ. 


of  the  retina  may  be  decomposed,  and  it  takes  the  retina  longer  to 
come  to  a  state  of  equilibrium.  The  effect  of  the  larger  areas, 
however,  cannot  be  explained  by  such  a  simple  process.  In  the 
latter  case  there  is  only  an  increase  in  the  number  of  retinal 
elements  stimulated,  and  not  a  greater  stimulation  of  any  one 
element.  An  explanation  of  the  greater  durations  with  the 
large  areas  must  probably  be  looked  for  in  another  direction. 
In  the  threshold  experiments  the  same  effect  was  noticed,  and 
it  was  concluded  that  the  cause  was  mental.  In  this  case, 
when  the  after-image  is  fading,  a  small  light  is  hard  to  notice, 
and  the  large  one,  with  its  numerous  points  for  the  shifting  of 
the  attention,  is  easy  to  keep  in  view.  The  different  durations 
for  the  large  and  the  small  areas  is,  accordingly,  only  an  ap- 
parent one. 

The  relative  effects  of  the  physical  units  are  shown  in  the 
accompanying  curves,  which  are  constructed  from  the  averages 
of  subjects  H,  K  and  F.  The  results  from  these  three  ob- 
servers only  were  used  because  a  slight  variation  due  to  the 
fourth  subject  would  disturb  the  common  point  of  coincidence 
in  the  three  series.  Fig.  13  represents  the  results  of  actual  time 
of  the  after-image,  while  Fig.  14  shows  the  results  for  the  total 
time. 

Such  a  simple  relation  as  was  found  in  the  threshold  ex- 
periments between  the  units  of  physical  energy  seems  not  to 


ON  AFTER-IMAGES.  27 

hold  here.  This  is  not  surprising  when  it  is  considered  that  the 
longer  time  that  the  image  is  in  view  gives  greater  opportunity 
for  outside  influences  to  affect  the  process. 


sec. 

60. 


DwawSE. 


m 


The  constancy  of  the  relation  of  the  actual  to  the  total 
duration  should  be  noted.  The  average  proportion  is  76 : 
100,  while  the  variation  for  this  is  about  3. 

Quality  of  Light. — If  a  spectrum  projected  on  a  wall  is 
looked  at  for  some  time  and  an  after-image  obtained  therefrom, 
it  will  be  noticed,  besides  the  qualitative  differences,  that  the 
image  of  the  central  portion,  the  yellow  and  green  parts,  lasts 
relatively  longer  than  that  of  the  red  or  violet  ends.  This  has 
sometimes  been  considered  as  evidence  that  the  central  portion 
of  the  spectrum  is  of  greater  intensity  than  the  ends.  This 
hypothesis  seems  at  first  sight  to  be  the  correct  one  ;  but  at  times 
it  has  not  been  sufficiently  considered  by  investigators. 

Bidwell x  notes  that  while  the  after-image  obtained  from  the 

1  Op.  cit.,  p.  140. 


5.    /.    FRANZ. 


whole  spectrum  was  violet,  he  could  get  no  after-image  from 
the  red  and  the  violet  of  the  spectrum.  This  fact  he  attempts 
to  explain  without  considering  the  intensity  differences,  and  he 
uses  the  observation  to  prove  that  the  image  is  due  to  an  ex- 
citation of  the  violet  nerve-fibers  only.  However,  the  accuracy 
of  Bidwell's  observation  has  been  questioned  by  Hess,  who- 
found  that  an  image  was  produced  by  red  lights  whose  wave- 
length was  less  than  623  //. 

Charpentier '  found  that  the  persistance  des  impressions 
retinniennes  was  affected  by  the  quality  of  the  light ;  but 
though  he  believed  the  difference  due  to  the  differences  in  the 
intensity  of  the  stimuli  rather  than  to  the  quality  of  the  light,  his 
results  were  not  reported  in  full. 

In  his  work  with  transmitted  light,  Titchener 2  used  various 
colored  gelatins,  which  resulted  generally  in  showing  a  longer 
duration  for  yellow  than  for  red  light.  In  these  experiments, 
however,  the  relative  intensities  were  not  the  same,  and  in  this 
connection  little  value  attaches  to  the  results.  The  results 
obtained  by  Titchener  for  effects  of  other  colors — violet,  green, 
etc. — are  incomparable  for  the  same  reason. 

I  have  attempted  to  exclude  the  factor  of  illumination- 
differences  by  using  gelatin  films  whose  absorbent  powers  were 
known.  The  light  was  so  arranged  for  each  gelatin  that  the 
intensity  was  equal  for  all.  The  following  table  shows  the 
results  for  stimulation  of  5  seconds  and  an  area  of  1  sq.  cm.  : 

Table  XVIII. 


V. 

B. 

G. 

Y. 

O. 

R. 

Duration  in 
seconds. 

24 

26 

30 

24 

27 

22 

A.  Var. 

6 

5 

3 

5 

7 

6 

Ten  determinations  were  made  with  each  kind  of  light.  On 
the  first  line  will  be  found  the  average  duration  under  the 
various  conditions ;  and  below,  the  average  variations  for  each 
series  of  experiments.     It  will   be   noticed  that  practically  no 

1  C.  Rend,  de  la  Soc.  de  Biol.,  Ser.  8,  Vol.  IV.,  p.  92,  1887. 

2  Op.  cit. 


ON  AFTER-IMAGES. 


29 


difference  is  found  to  exist  between  the  after-image-producing 
power  of  any  colors.  The  variations  make  an  overlapping  of 
the  times  in  such  a  way  as  to  show  that  the  slight  differences 
noticed  at  first  in  the  table  are  only  apparent. 

In  this  connection  it  would  be  interesting  to  test  individuals 
who  are  partially  or  totally  color-blind.  Tests  on  such  a  class 
would  probably  be. the  only  valid  ones  as  indicating  the  part 
played  by  the  quality  and  by  the  intensity  of  the  light. 

Parts  of  the  Retina. — -Purkinje,  Aubert  and  Exner  have 
noted  independently  that  light-sensations  continued  longer  in  the 
fovea  than  on  the  periphery  of  the  retina.  These  observations 
have  been  disputed  by  more  recent  investigators,  however. 
The  experiments,  dealing  only  with  the  phenomenon  of 
visual  persistence,  do  not  indicate  what  might  be  expected  for 
other  similar  phenomena,  particularly  the  longer  continua- 
tions, such  as  after-images.  In  view  of  the  fact  that  we  can 
see  less  clearly  with  the  peripheral  portions  of  the  retina,1  and 
that  these  parts  are  more  easily  fatigued,2  it  seemed  probable 
that  the  after-image  would  also  be  less  distinct  and  would  con- 
tinue a  shorter  time.  The  supposition  was  proved  correct  by 
the  results  of  the  experiments. 

Each  of  seven  points  along  the  inner  side  of  the  right  eye 
were  stimulated  with  a  stationary  light,  and  the  total  and  the 
actual  times  of  duration  of  the  after-image  were  noted.  As 
stimulus,  I  used  a  small  lamp  with  an  intensity  of  about  y2  c.  p. 

Table  XIX. 


1  See  Aubert,  Grundziige  d.  Physiol.  Optik,  1876,  p.  539  ft'. 
2 Ibid.,  p.   545. 


Angular  distance 
from  fovea. 

0° 

15° 

300 

45° 

60° 

75° 

900 

Total         Aver. 

40 

32 

10 

0? 

0 

0 

0 

Duration  A.  Var. 

S 

9 

4 

- 

- 

- 

- 

No.  Exp. 

10 

10 

10 

5 

5 

2 

2 

Actual       Aver. 

35 

24 

8 

0? 

0 

0 

0 

Duration  A.  Var. 

7-5 

9 

5 

— 

- 

— 

- 

No.  Exp. 

10 

10 

10 

5 

5 

2 

2 

30  5.    /.   FRANZ. 

This  was  looked  at  for  about  5  seconds,  and  the  angular  dis- 
tance subtended  was  about  two  degrees.  The  light  was  at  all 
times  at  a  constant  distance  from  the  eye  (retina),  viz.,  30  cm. 
The  table  on  page  29  shows  the  results  of  the  experiment  upon 
each  of  the  points,  o°  being  the  fovea. 

Not  only  were  there  differences  in  the  duration  of  the  after- 
image, but  the  fainter  appearance  of  a  sensation  toward  the 
periphery  was  duplicated  in  the  after-images.  No  variations  in 
color  of  the  after-image  corresponding  to  the  different  points 
tested  was  apparent. 

This  gradual,  but  finally  absolute,  lack  of  ability  to  distin- 
guish an  after-image  is  probably  due  to  several  factors.  Aubert 
mentions  that  the  periphery  is  more  easily  fatigued  than  the 
fovea ;  but  he  does  not  seem  to  consider  that  this  may  be  partly 
mental  and  not  entirely  physiological.  From  observations  made 
during  the  progress  of  the  experiments  it  seems  likely  that  the  in- 
ability to  attend  to  these  things  not  in  its  immediate  vicinity  is  the 
primary  reason  for  the  lack  of  images  toward  the  periphery, 
and  for  the  long  durations  at  the  fovea.  At  450  it  was  felt  at 
times  that  an  image  might  be  present,  but  so  indistinct  that  it 
was  impossible  to  give  any  definite  answer.  In  the  table  these 
results  are  accordingly  given  as  o  (?). 

A  similar  but  more  extended  set  of  experiments  upon  the 
quality  as  well  as  the  duration  would  be  important  for  testing 
color  theories,  but  this  point  of  view  was  not  considered  till  the 
present  series  had  been  finished. 

Individual  Differences. — Throughout  the  foregoing  pages 
one  or  more  individuals  have  been  considered  as  giving  typical 
results.  That  this  is  not  strictly  true,  however,  has  been  fully 
recognized ;  and  from  the  material  at  hand  I  have  endeavored 
to  discover  some  of  the  individual  conditions  upon  which  the 
duration  depends.  The  numerous  difficulties  in  such  a  study 
are  evident :  each  individual  is  not  the  same  at  any  two  mo- 
ments, and  various  distracting  thoughts  cannot  be  excluded. 

During  the  past  few  years  there  has  been  made  at  the  Co- 
lumbia laboratory  a  series  of  mental  and  physical  tests.  Among 
these  was  one  on  after-images,  in  which  the  duration,  the  color, 
etc.,  were  noted.     One  great  defect  of  these  measurements  is  to 


ON  AFTER-IMAGES.  31 

be  found  in  the  fact  that  there  were  many  experimenters  instead 
of  one,  making  methods  of  experimenting  and  of  recording  to 
some  extent  dissimilar.  In  general,  however,  the  results  are 
fairly  comparable,  and  are  as  accurate  as  the  method  of  correla- 
tion used  in  this  case.  The  individual  differences  of  recording 
had  little  influence  upon  the  measurement  of  the  total  time ;  but 
by  some  the  actual  time  with  the  various  fluctuations  were  not 
given  at  all.  For  this  reason  only  the  total  time  can  be  used  as 
a  basis  for  comparison. 

The  stimulus  was  a  cross,  with  arms  1  sq.  cm.,  illumined  by 
a  100  c.  p.  incandescent  lamp  in  a  box.  The  lamp  was  30  cm. 
from  a  plate  of  ground  glass,  which  was  used  to  make  the  cross 
of  uniform  brightness.  The  subject  was  seated  in  a  semi-dark 
room,  and  no  adaptation  time  was  needed.  The  cross  was 
shown  for  ten  seconds  and  then  shut  off  by  a  screen.  The  sub- 
jects, having  closed  and  covered  the  eyes  without  pressing  upon 
them,  informed  the  experimenter  of  the  various  phases  as  they 
occurred,  and  these  were  recorded  as  well  as  possible.  Gener- 
ally two  trials  were  given. 

The  tests  chosen  for  comparison  with  the  results  of  the  after- 
image test  were  sharpness  of  vision,  color-vision  and  imagina- 
tion-type. These  were  so  chosen  because  they  seemed  closely 
allied  to,  or  were  thought  to  have  an  influence  upon,  the  after- 
image. It  would  have  been  interesting  to  compare,  in  addition, 
visual  and  auditory  memory,  time  of  reaction  to  visual  and  to 
auditory  stimuli,  etc. ;  but  at  that  time  such  tests  were  not  in- 
cluded in  the  anthropometric  series. 

Sharpness  of  vision  was  tested  with  the  Galton  eyesight- 
tester.  This  is  an  instrument  having  at  certain  distances  cards 
of  numerals  in  diamond  type  which  are  to  be  read  by  the  sub- 
ject, who  looks  through  a  hole  in  a  screen.  The  type  was 
illumined  by  a  100  c.  p.  incandescent  lamp  at  a  distance  of  one 
meter  from  the  card,  which  was  37  cm.  from  the  eyes.  The 
normal  distance  for  reading  correctly  eight  or  more  out  of  ten 
numerals  was  for  each  eye  42-52  cm. 

Color-vision  was  tested  with  spools  of  colored  wools ;  but 
this,  being  only  a  rough-and-ready  method  of  detecting  color- 
blindness, did  not  show  defects  which  did  not  amount  to  blind- 


32  S.  /.   FRANZ. 

ness.  Only  six  color-defectives  were  found,  and  their  after- 
images may  be  paralleled  by  numerous  other  observers  who 
were  considered  nearer  normal.  The  results,  accordingly, 
show  no  unusual  variations. 

The  imagination-type  was  discovered  by  means  of  the  fol- 
lowing series  of  questions,  part  of  which  are  the  classic  ones  of 
Galton :  Think  of  your  breakfast-table  as  you  sat  down  to  it 
this  morning ;  call  up  the  appearance  of  the  table,  the  dishes 
and  food  upon  it,  the  persons  present,  etc.  Then  write  answers 
to  the  following  questions :  Are  the  outlines  of  the  objects  dis- 
tinct and  sharp?  Are  the  colors  bright  and  natural?  Where  does 
the  image  seem  to  be  situated?  In  the  head?  Before  the  eyes?  At 
a  distance?  How  does  the  size  of  the  image  compare  with  the 
actual  size  of  the  scene  ?  Can  you  call  to  mind  better  the  face 
or  the  voice  of  a  friend?  When  violin  is  suggested,  do  you 
first  think  of  the  appearance  of  the  instrument  or  the  sounds 
made  when  it  is  played?  Can  you  call  to  mind  natural  scenery 
so  that  it  gives  you  pleasure?  Music?  The  taste  of  fruit?  Have 
you  ever  mistaken  a  hallucination  for  a  perception  ;  e.  g\,  appar- 
ently heard  a  voice  or  seen  a  figure  when  none  was  present? 

Of  the  subjects  who  answered  the  questionnaire,  59^  would 
be  considered  as  of  a  distinct  visual  type,  ^j4fo  were  distinctly 
auditory  and  36^/0  would  be  considered  of  mixed  type. 

For  comparing  these  results  with  the  duration  of  the  after- 
image, only  the  simple  method  of  correlation  was  used.  The 
results  of  the  subjects  that  were  considered  as  having  hyper- 
normal,  normal  or  subnormal  vision  were  grouped  and  the  aver- 
age duration  of  the  after-image  was  calculated  ;  a  similar  method 
was  used  for  the  imagination-type  correlation.  This  method  is 
not  above  criticism ;  but  it  is  sufficiently  accurate  to  show  cer- 
tain tendencies  and  influences.  The  results  for  the  accuracy 
of  vision  in  the  right  and  left  eyes,  and  for  the  type  of  imagina- 
tion, are  summarized  in  the  following  tables. 

Objection  may  be  made  to  this  method  of  correlation,  to  the 
grouping  the  subjects  and  the  use  of  one  or,  at  most,  two  ex- 
periments, as  giving  a  typical  value  for  the  individual.  The 
objections  would  apply  to  all  anthropometric  series,  and  it  has 
weight  only  when  one  attempts  to  apply  results  to  individuals. 


ON  AFTER-IMAGES. 


33 


It  is  only  in  considering  an  individual  as  of  a  class  that  the 
figures  have  a  value. 

Table  XX. 


Acuteness  of  vision, 
right  eye. 

No.  of 
cases. 

No.  of 
exper. 

Av.  durat.  of  after- 
image in  sees. 

Av. 

Percentage  of 
times  no  image 

0-37  cm. 
Subnormal. 
44-52  cm. 
normal. 
61-85  cm. 
Hyper  normal. 

37 
63 
35 

57 
92 

59 

26 
37 
37 

49 

27 
25 

Table  XXI. 


Acuteness  of  vision, 
left  eye. 

No.  of 
cases. 

No.  of 
exper. 

Av.  durat.  of  after- 
image in  sees. 

Av. 

Percentage  of 

times  no  image 

was  seen. 

0-37 

subnormal. 

44-52 

normal. 

61-72 

hyper  normal. 

37 
76 
26 

57 
106 

42 

32 
34 
36 

46 
26 
26 

Table  XXII. 


Mental   Type. 

No.  of 
cases. 

No.  of 
exper. 

Av.  durat.  of  after- 
image in  sees. 

Av. 

Percentage  of 
times  no  image 

Visual. 
Mixed. 
Audile. 

77        I      T2i 

47              65 

6                8 

36 
37 
16 

26 
32 
63 

Probably  the  most  interesting  fact  to  be  noted  in  the  tables 
is  not  that  the  duration  is  longer  for  the  visual  type,  or  for  the 
man  with  strong  eyes,  but  that  in  these  classes  the  total  number 
of  times  when  no  image  was  noticed  is  small  compared  with  the 
percentage  for  the  audible  type,  or  for  those  having  weak  eyes. 

Granting  that  these  variations  are  true  criteria  of  the  differ- 
ent classes,  an  explanation  must  be  sought.  It  is  easy  to  see  why 
an  individual  of  a  visual  type  might  have  an  after-image  of 
longer  duration  than  one  of  an  audile  type.  Accustomed,  as  he 
is,  to  note  principally  those  phenomena  which  appeal  to  his 
eyes,  practice  enables  him  to  select  the  visual  stimuli  and  to  at- 
tend to  these  even  if  of  little  intensity.  Accordingly,  an  after- 
image in  its  final  stages,  when  very  dim,  would  be  noted  by 


34  5.   I.   FRANZ. 

him ;  whereas  an  auditory  type  of  individual  might  neglect 
this  minimal  amount  of  sensation.  A  similar  hypothesis  would 
explain  why  audiles  should  see  less  images  than  visuals.  Many 
of  the  images  are  undoubtedly  of  little  intensity,  and  they  would 
be  unnoticed  by  those  individuals  unaccustomed  to  note  small 
visual  differences.  This  would  reduce  the  differences  to  a 
question  of  the  attention  or  of  habit,  rather  than  to  a  physio- 
logical process ;  and  this  explanation  is  in  accord  with  what  was 
noted  above  in  the  experiments  upon  the  increase  in  the  area  of 
stimulation. 

In  a  similar  series  made  upon  a  class  of  seven  boys  by  Dr. 
F.  B.  Brandt  this  explanation  is  further  confirmed.  Using  a 
stimulation  of  i  sq.  cm.  of  the  northern  sky  fixated  for  15 
seconds  at  a  distance  of  30  cm.,  Dr.  Brandt  found  that  those 
boys  who  held  the  highest  rank  in  school-work,  those  who  had 
learned  to  fix  their  attention,  could  see  the  after-image  for  a 
longer  time  than  those  who  were  dull  and  inattentive.  The 
attention  in  this  case  may  have  been  largely  influenced  by 
'  interest,'  but  the  general  result  and  conclusions  are  the  same. 
The  '  bright'  boy  is  he  who  is  interested  in  each  novel  experi- 
ence ;  is  he  who  attends  with  all  his  might  to  what  is  immediately 
present.  During  the  course  of  the  experiments  this  view  was 
clearly  indicated  by  a  study  of  the  actions  of  the  subjects.  A 
and  B  had  been  intellectual  rivals  throughout  their  school- 
work,  and  when  B  found  that  A  was  having  after-images  of 
longer  duration  than  he,  his  interest  flagged  and  it  was  difficult 
to  persuade  him  to  continue.  While  there  are  some  variations 
in  the  relation  between  scholarship  and  the  duration  of  the  after- 
image, the  results  show  that  the  three  boys  of  excellent  ability 
very  easily  outrank  the  other  four.  The  following  table  gives 
the  results  of  ten  experiments  upon  each  subject. 

In  conjunction  with  the  attention,  imagination  undoubtedly 
plays  an  important  role  in  these  phenomena.  Miss  Washburn 
has  noted  that  she  was  able  to  control  voluntarily  the  colors  of  the 
after-image  ;J  and  in  addition  she  mentions  that  when  she  visual- 
ized a  color  and  tried  to  force  the  after-image  to  take  this  quality, 

JM.  F.  Washburn,  Subjective  Colors  and  the  After-image ;  Mind,  N.  S., 
VII.,  25-34,  l899- 


ON  AFTER-IMAGES. 

Table  XXIII. 


35 


Subject. 

Scholarship  grade. 

Duration'of  after- 
image in  seconds. 

Average  variations. 

A. 

96% 

320 

36 

B. 

95 

169 

64 

C. 

92 

267 

65 

D. 

82 

60 

26 

E. 

81 

69 

51 

F. 

80 

13s 

40 

G. 

79 

42 

31 

'  the  image  of  the  visualized  color  was  brought  on  sooner,' 
and  '  held  longer  than  usual.'  Miss  Washburn  concludes,  as 
I  do  above,  that  the  image  is  largely  influenced  by  the  attention, 
and  that  perhaps  the  imagination  is  to  be  explained  as  the  call- 
ing the  attention  to  any  part  of  the  image ;  the  color  is  there, 
but  it  remains  unnoticed  till  we  are  prepared  for  it.  In  this  way 
the  after-image  would  also  appear  to  be  in  view  a  longer  time. 

Miscellaneous. — Besides  these  factors,  other  influences  affect 
the  duration  of  the  phenomena.  Helmholtz  l  noted  that  if  an 
electric  current  be  sent  through  the  eye  and  the  optic  nerve, 
the  image  is  changed  in  character  and  the  time  of  duration  is 
(probably)  shortened.  This  change  is  analogous  to  that  oc- 
curring when  the  eye  is  stimulated  with  light  while  an  image  is 
in  view.  Exner2  found  that  the  retinal  circulation  and  pressure 
on  the  ball  of  the  eye  influenced  the  duration.  Brewster3  in 
1834  n°ted  that  a  smart  blow  on  the  head  would  stop  an  image 
in  its  course.  Similar  results  were  obtained  from  my  subjects, 
but  sometimes  the  image  would  also  seem  of  less  intensity. 
In  the  morning,  when  the  eyes  were  fresh,  the  image  lasted 
about  30^  longer  than  in  the  evening  or  when  fatigue  was  ap- 
parent. 

All  these  factors  may  be  resolved  into  an  influence  of  the 
attention  on  the  one  hand,  or  of  the  physiological  process  in  the 
sense-organ  on  the  other.  Either  explanation  would  suffice  ; 
but  it  seems  that  both  should  be   considered.     In   conjunction 

1  Physiol.  Optik,  509. 

2  Ueber  die  Funktionsweise  d.  Netzhautperipherie   und  den  Sitz  der  Nach- 
bilder;  Archiv  f.  Ophthal.,  XXXII.  (2),  233-252,  1886. 

3  Accounts  of  Two  Experiments  on  Accidental  Colors  ;  Philos.  Mag.,  3  Ser., 
IV.,  May,  1834. 


36  S.    f.   FRANZ. 

with  the  material  of  the  foregoing  paragraphs  it  seems  likely 
that  the  mental  element  is  very  great — that  is,  the  blow  on 
the  head  or  the  general  fatigue,  the  pressure  on  the  eyeball, 
etc.,  are  influences  inhibiting  the  focussing  of  the  attention 
upon  the  after-image.  Particularly  is  this  the  case  when  these 
factors  are  brought  in  in  an  artificial  manner  that  their  effect  may 
be  observed.  Voluntary  winking,  where  the  attention  must  be 
turned  away  from  the  sensation  to  the  movement,  shows  the 
same  effect. 

When  the  stimulus  was  not  attended  to,  when  the  subject 
was  distracted  with  a  continued  conversation,  while  all  distract- 
ing influences  ceased  with  the  end  of  the  stimulus,  the  duration 
and  the  intensity  of  the  after-image  seemed  not  affected.  When 
however,  distracting  sensations,  sounds  and  conversation  were 
introduced  during  the  progress  of  the  after-image  the  attention 
was  distracted  sufficiently  to  cause  an  apparent  shortening  in 
the  duration  of  the  after-image  by  about  40%. 

Summary. — The  after-image,  particularly  in  its  duration,  if 
affected  by  many  mental  and  physical  conditions.  The  most 
influential  of  these  seems  to  be  the  mental  attitude  of  the  sub- 
ject. If  he  looks  intently  for  the  after-image,  if  he  is  accus- 
tomed to  note  visual  changes,  the  image  will  apparently  remain 
longer  than  if  his  attitude  is  more  motor  or  auditory.  When 
the  attention  is  directed  in  its  greatest  intensity  to  the  after- 
image, either  from  habit,  because  one  is  visual-minded,  or 
voluntarily,  the  duration  is  about  one-third  longer  than  when 
the  attention  is  not  so  directed.  Various  physical  conditions — 
e.  g.,  having  the  image  on  the  periphery  of  the  retina,  physical 
fatigue,  etc.,  which  give  similar  results — may  also  be  con- 
sidered in  this  class.  Of  the  physical  light-changes,  the  most 
influential  seems  to  be  the  time  of  stimulation.  This  corre- 
sponds to  what  was  found  in  the  experiments  on  the  threshold 
and  latent  period.  The  area-  and  intensity-changes  also  have 
effects  similar  to  what  was  found  in  the  experiments  upon  thresh- 
old and  latent  period ;  but  no  general  relation  of  these  effects 
seems  to  be  discovered.  Figures  13  and  14  show  composites  of 
the  average  results  from  Tables  IX.—  XVII.,  and  from  them  the 
relative  effects  of  the  physical  units  may  be  noted.     Contrary  to 


ON  AFTER-IMAGES.  37 

the  generally  accepted  view,  variations  in  quality  (color)  seem 
to  have  little  effect  unless  there  is,  as  usual,  a  change  in 
intensity. 

Section  5.  Fluctuations. — In  all  discussions  of  the  after- 
image one  of  the  questions  that  has  been  largely  neglected  is 
that  regarding  the  fluctuations  from  positive  to  negative,  its 
disappearances  and  reappearances.  Immediately  upon  the  close 
of  the  stimulus  an  after-image  due  to  the  continuation  of  the 
light  is  seen.  A  vacant  period  (the  latent  time,  see  Sec.  3)  fol- 
lows, and  then  an  image  which  in  turn  fades  away  into  another 
color  or  disappears  and  reappears  several  times.  This  fluctu- 
ation was  noted  by  R.  W.  Darwin,1  and  has  been  variously  as- 
cribed to  many  physiological  causes  connected  with  the  sense- 
organ — to  eye-movements,  to  winking,  to  pressure  on  retina, 
etc.  During  the  progress  of  the  present  work  I  endeavored  to 
note  the  various  effects  of  these  conditions.  Most  of  the  experi- 
ments were  made  in  a  dark-room,  and  the  image  was  looked  for 
with  eyes  open.  Pressure  on  the  eyeball  was  thus  prevented, 
but  the  fluctuations  continued.  Winking,  if  unconsciously  done, 
disturbed  the  image  momentarily,  but  had  no  lasting  effect. 
However,  if  consciously  done  (i.  e.,  when  the  attention  was  di- 
rected away  from  the  after-image  to  the  movements  of  the  eye- 
lids), the  image  seemed  lost  for  an  appreciable  time.  The 
numerous  involuntary  eye-movements  could  not  be  controlled. 
The  eyes  tended  to  follow  the  image,  which  would  remain  in 
view  while  these  movements  were  not  inhibited.  If  consciously 
prevented,  if  the  eyes  were  brought  back  to  what  appeared  to 
be  their  normal  position,  if  the  eyes  were  consciously  rotated 
while  the  phenomena  lasted,  the  image  would  disappear,  only  to 
reappear  if  consciously  brought  back — i.  e.,  if  the  attention 
was  again  directed  to  the  organ  of  sensation  rather  than  to  the 
movements  of  this  organ.  From  these  observations  of  my  sub- 
jects and  myself  I  am  inclined  to  believe  many  of  the  fluctua- 
tions are  mental  in  character,  just  as  we  have  seen  the  duration 
to  be  largely  a  mental  matter.     In  other  words,  the   various 

1  R.  W.  Darwin,  New  Experiments  upon  the  Ocular  Spectra  of  Light  and 
Colours;  Phil.  Trans.,  LXXVL,  313-348,  1786. 


3S  S.   f.   FRANZ. 

phases  are  influenced  more  by  the  mental  attitude  of  the  subject 
than  by  the  physiological  condition  of  the  retina. 

Helmholtz  noted  that  violent  movements  of  the  body 
changed  the  character  of  the  after-image  from  positive  to  nega- 
tive,1 and  that  an  electric  current  sent  through  the  optic 
nerve  changed  the  intensity  of  the  after-image  present.2  This 
may  also  be  seen  to  agree  with  the  attention  hypothesis. 

In  some  respects  the  fluctuations  of  the  image  arising  from 
stationary  stimuli  are  analogous  to  those  noted  by  Bidwell 3 
and  by  Charpentier.4  However,  the  images  noted  by  these  in- 
vestigators were  so  complex  that  it  seems  uncertain  whether 
we  are  dealing  with  the  same  phenomena.  The  Bidwell  recur- 
rent images  are  explained  satisfactorily  upon  physiological 
grounds  ;  but  in  repeating  the  experiments  I  was  led  to  believe 
that  at  least  the  brightness  of  the  various  bands  was  due  in  large 
measure  to  the  fixation  of  the  attention. 

The  changes  in  the  physical  stimuli  seemed  to  have  no 
effect  in  the  present  case.  The  after-images  when  they  lasted 
longer  did  not  have  more  fluctuations,  but  the  various  phases 
remained  for  longer  times.  Such  a  great  variation  in  the  num- 
ber and  in  the  character  of  the  fluctuations  was  noted  that  it 
seemed  impossible  that  any  general  relation  held  true.  Op- 
posed to  these  results,  however,  are  observations  by  Helmholtz5 
and  by  Miss  Washburn.6  Helmholtz  noted  that  the  duration 
of  the  stimulus  had  an  influence  upon  the  negative  as  well  as 
upon  the  positive  phases ;  but  he  only  makes  this  general  state- 
ment without  further  detail.  Miss  Washburn  also  observed  vari- 
ations in  the  fluctuations  due  to  intensity-changes  of  the  stimulus. 

Section  6.  Qualitative  Changes. — As  one  of  the  most 
difficult  and  most  striking  of  the  phenomena  to  be  explained, 
the  color-changes  of  the  after-image  have  attracted  a  great 
amount  of  attention.  But  throughout  the  various  accounts  of 
the  color-changes  it  seems  to  have  been  assumed  that  the  ex- 
perimenter was  normal,  and  what  occurred  in  his  eyes  would  or 

1  Physiol.  Optik,  p.  504.  *Op.  cit. 

2  Ibid.  5Op.  cit.,  p.  505. 

3  Op.  cit.  6  Op.  cit.,  p.  27. 


ON  AFTER-IMAGES.  39 

should  occur  to  another,  if  he  saw  the  same  light.  This 
belief  has  led  in  some  measure,  I  fancy,  to  the  disagreements 
regarding  the  phenomena.  The  individual  element  seems  not 
to  have  been  considered  by  any  one.  In  the  experiments  noted 
in  Section  4  it  was  found  that  the  various  observers  differed 
greatly  in  their  account  of  the  color-changes ;  and,  moreover, 
an  individual  did  not  always  see  the  same  colored  after- 
image when  stimulated  with  the  same  light.  At  one  time,  all 
the  images  would  be  seen  as  light  alone,  at  another,  they  ap- 
peared gray  or  reddish.  This  change  appeared  with  all  my  sub- 
jects. The  difference  was  also  noted  by  R.  W.  Darwin,1  who 
considered  the  changes  as  due  to  his  making  too  many  experi- 
ments at  one  time,  his  eye  being  '  not  quite  free  from  the 
spectra  of  the  colours  previously  attended  to.'  The  difference 
of  individuals  is  naively  noted  by  Miss  Washburn  in  her  recent' 
paper.  In  this  paper  she  says  her  subjects  had  to  be  drilled  to 
see  a  normal  image  («'.  e.,  like  her  own).2 

From  the  after-image  experiments  in  the  series  of  anthro- 
pometric tests  (see  Sec.  4),  it  was  found  that  "  the  after-image, 
when  first  seen,  was  sometimes  positive  and  sometimes  nega- 
tive :  and  the  colors  varied  greatly,  being  distributed  in  the  first 
place  as  follows  :  negative  or  dark,  33^  ;  light  or  white,  29.4  ; 
blue,  13.7;  purple,  9.8;  green,  5.9;  yellow,  3.9;  red,  2.0; 
miscellaneous,  2.0." 3 

Various  trials  were  made  of  physical  conditions  to  discover 
whether  or  not  the  change  was  due  to  these ;  but  for  the  condi- 
tions tested  (position  of  eyes  in  relation  to  light,  accommodation, 
adaptation,  changes  in  intensity,  duration  and  area  of  stimulus), 
nothing  definite  was  discovered. 

Fechner  found  that  various  durations  of  the  stimulus  gave 
different  colored  after-images,  but  I  found  no  such  definite 
relation  with  the  comparatively  long  times  used  by  me.  The 
change  in  quality  of  the  after-image  under  any  one  condition — 
e.g.,  5  seconds — seemed  as  great  as  that  observed  between  5 
seconds  and  1  second  stimulation.      The  only  uniform  change 

!Op.  cit. 

2  Op.  ct.,  p.  27. 

3  Cattell  and  Farrand,  Physical  and  Mental  Measurements  of  the  Students  of 
Columbia  University;   Psycho,.  Rev.,  III.,  618-648,  1896.   See  especially  p.  645. 


40  5.   I.   FRANZ. 

noted  was  that  with  the  greater  intensities  and  the  longer  stim- 
ulations the  after-image  appeared  oftener  as  negative.  With 
medium  intensities  such  as  I  used  this  result  is  almost  never 
noted ;  but  the  change  is  clearly  brought  out  in  the  results  of 
the  experiments  upon  threshold  (where  only  \  of  ifo  were  nega- 
tive) compared  with  the  results  upon  duration  (where  fully  8of> 
of  the  images  had  negative  phases). 

The  quality  (or  intensity)  of  the  image  was  changed  by 
Helmholtz  by  sending  an  electric  current  through  the  eye.  "  If 
one  has  developed  in  the  eye  a  negative  after-image  and  then 
sends  an  electric  current  through  the  eye  and  the  optic  nerve, 
*  *  *  the  negative  after-image  becomes  darker,  and  if  an 
image  is  just  on  the  borderland  between  positive  and  negative, 
it  can  be  made  negative  by  sending  a  current  through  the  eye."1 
This  change  is  somewhat  analogous  to  what  occurs  when  an 
after-image  is  produced  and  external  light  is  then  admitted  to 
the  eye. 

I  can  attempt  no  explanation  for  these  individual  differences. 
The  suggestion  has  been  made  that  the  differences  may  be  due 
to  a  varied  sensitiveness  of  the  retina  to  light-waves  of  different 
lengths.  Such  an  explanation  seems,  however,  too  hypothetical 
when  one  considers  the  great  differences.  One  subject  saw 
the  after-image  always  as  green ;  to  another  it  was  always  red ; 
to  a  third  it  varied  in  the  white-black  series.  The  explanation 
could  not  be  stretched  in  any  case  to  cover  the  change  in  an  in- 
dividual under  practically  the  same  conditions.  The  importance 
of  these  differences  for  theories  of  color-vision  will  readily  be 
recognized.  If  later  experiments  are  made,  a  most  careful 
study  of  each  subject's  color-vision  should  also  be  made. 

The  control  of  the  color  of  the*  after-image  should  again  be 
noted.  Miss  Washburn  attempted  to  control  the  color-changes, 
but  concludes  that  the  control  is  not  real,  but  apparent.  In  her 
case  the  attention  intensified  certain  colors  that  were  present, 
but  which  would  be  unnoticed  under  ordinary  circumstances. 
Such  an  explanation  of  the  effects  of  attention  probably  could 
not  be  made  to  include  the  variation  of  one  individual  from 
another.     The  problem  needs  more  detailed  and  careful  study. 

1  Op.  cit.,  p.  509. 


ON  AFTER-IMAGES.  41 

An  attempt  to  change  voluntarily  the  character  of  the  after- 
image from  positive  to  negative  has  been  uniformly  unsuccess- 
ful in  my  own  case  ;  Miss  Washburn  seems  to  indicate  that  this 
was  done  by  all  her  subjects,  and  it  may  be  that  being  a  poor 
visualizer  accounts  for  my  inability. 

Section  7.  Space-relations. — In  his  characterization  of 
the  difference  between  after-images  and  the  images  of  the  im- 
agination, Fechner  considers  as  one  of  the  greatest  differentia 
the  spatial  character  of  the  phenomena.  The  after-image  is, 
according  to  him,  of  two  dimensions  only,  while  the  images  of 
the  imagination  are  tridimensional.  This  statement  has  not 
often  been  disputed,  but  there  seems  little  truth  in  it  as  a  state- 
ment of  universal  fact.  Professor  Hyslop  found  that  if  a 
"  picture  hanging  obliquely  on  the  wall,  say  thirty  degrees, 
more  or  less,  and  I  look  at  it  while  lying  on  a  bed  or  lounge 
and  then  look  at  the  wall  vertically  near  me,*  *  *  I  notice  that  the 
after-image  does  not  lie  in  the  plane  of  the  wall,  but  in  the  same 
■position  relatively  to  the  plane  of  vision  as  in  its  real  position.^ 
In  other  words,  we  have  here  a  case  where  the  after-image 
was  distinctly  opposed  in  character  to  what  Fechner  said  we 
have  it. 

In  view  of  these  contradictory  statements,  it  seemed  advis- 
able to  test  the  matter  more  thoroughly,  using  several  subjects. 
A  few  simple  experiments  were  devised  for  the  purpose.  The 
subjects,  naive  as  to  the  purpose  of  the  experiment,  were  asked  to 
gaze  fixedly  for  fifteen  seconds  at  one  corner  of  a  highly  colored 
rectangular  block,  and  to  project  the  resultant  after-image 
upon  a  white  wall  about  four  meters  distant.  Note  was  to  be 
made  of  size,  color,  etc.,  but  no  suggestion  was  given  regard- 
ing the  tridimensional  character.  Throughout  the  experiments 
the  eyes  were  open.  A  brightly  lacquered  round  resonator, 
two  brightly  colored  books  placed  like  a  V  with  the  apex  turned 
away  from  the  subject,  and  two  incandescent  lamps  placed  at  dif- 
ferent distances  from  the  eyes,  were  also  used  as  stimuli.  Some 
of  the  observations  with  the  various  stimuli  are  as  follows  : 

lJ.  H.  Hyslop,  Experiments  in  Space  Perception;  Psychol.  Rev.,  I.,  588, 


42  5.   /.    FRANZ. 

"  Projected  from  the  wall,  but  not  distinctly." 

"  Seemed  to  stand  back.     Solid." 

"  Some  suggestion  of  solidity,  but  no  definite  outline." 

"Perhaps  (?)  solid." 

"Vague  feeling  of  projection  from  wall." 

Most  of  the  results,  however,  seem  negative.  With  closed 
eyes,  on  the  other  hand,  the  image  became  more  nearly  like 
the  imagination-images ;  and  while  there  was  a  feeling  of  in- 
security, the  images  acquired  the  tridimensional  character  of  the 
mental  products.  This  may  be  due  (in  some  cases  it  was  due) 
to  the  superposition  or  to  the  strengthening  of  the  after-image 
by  the  imagination-image.  Here  we  must  also  consider  the 
effects  of  the  attention.  We  ordinarily  see  what  we  wish  to 
see,  and  the  numerous  questions  regarding  the  space-relations 
may  have  tended  to  suggest  to  the  subjects  the  answer  sought. 
With  some  subjects,  however,  no  amount  of  experiments  or 
suggestions  could  get  them  to  say  anything  beyond  that  the 
image  seemed  '  almost  solid.' 

If  my  own  attitude  is  to  be  considered  typical,  I  should  say 
that  the  image  appears  at  first  distinct  only  in  outline.  The  dif- 
ferences of  light  and  shade  which  we  note  in  the  original  solid 
object  do  not  appear  in  the  after-image  till  we  look  for  them, 
and  then  it  is  difficult  to  say  whether  the  differences  are 
really  due  to  the  imagination  or  whether  they  are  in  the  after- 
image. The  time  it  took  many  of  my  subjects  to  note  the 
third  dimensions  would  indicate  that  this  space-character  was 
'  read  into '  the  sensation  rather  than  given.1  This  much,  how- 
ever, seems  clear,  that  if  there  is  any  appearance  of  solidity 
connected  with  the  after-image,  it  is  not  nearly  so  evident  as  it 
is  in  sensation  or  in  imagination.  The  fact  that  the  image  is 
projected  by  us  into  space,  and  is  there  localized,  leads  one  to 
believe  that  the  idea  of  depth  is  not  wholly  wanting  in  the  ordi- 
nary after-image.  This  would  seem  to  class  the  phenomena 
rather  as  perceptions  than  as  sensations. 

1  Professor  Hyslop  advises  me  that  almost  seventy-five  per  cent,  of  the  stu- 
dents in  his  introductory  classes  fail  to  perceive  at  once  the  third  dimension  in 
stereoscopic  pictures.  Many  succeed  after  the  suggestion  has  heen  given,  but 
some  fail  to  obtain  the  usual  result  even  after  many  experiments. 


ON  AFTER-IMAGES.  43 

Stereoscopic  experiments,  in  which  the  sensation  was  one  of 
solidity,  gave  no  indication  of  apparent  solidity  in  the  resultant 
after-image. 

The  size  of  the  image,  as  is  well  known,  apparently  in- 
creases or  decreases  if  we  look  respectively  at  a  far  or  at  a  near 
object.  This  apparent  change  in  size  is  due  to  the  fact  that  a 
certain  portion  of  the  retina  is  concerned  with  the  seeing  of 
any  after-image,  and  the  amount  of  space  subtended  by  this 
portion  of  the  eye  increases  as  the  square  of  the  distance  of  the 
wall  or  screen.  When  projected  on  the  same  screen  the  size 
of  the  after-image  from  any  light  is  approximately  constant 
for  all  observers ;  the  few  variations  noted  are  probably  due  to 
hasty  judgments  or  poor  observations.  It  may  be  that  the  mov- 
ing of  the  eyes  during  the  stimulus  produced  in  these  cases  a 
larger  image,  more  of  the  retinal  elements  having  been  stimu- 
lated. A  curious  fact  in  connection  with  the  apparent  change  in 
size  is  that  if  a  long  image  is  projected  upon  a  receding  wall 
one  end  is  seen  wider  than  the  other.  A  more  wonderful  and 
more  puzzling  phenomenon  is  mentioned  by  James.  If  an 
after-image  is  projected  on  a  flat  surface  '  resembling  a  re- 
ceding screen,'  the  image  takes  on  the  form  it  would  have  if 
seen  under  similar  actual  conditions  just  noted.  Such  a  result 
points  strongly  to  mental  influences,  which  we  have  seen 
are  important  factors  in  all  conditions  of  the  after-image,  viz., 
attention  and  imagination.  The  experiment  makes  it  evident 
that  the  phenomena  are  closely  related  to  the  imagination  and 
to  perceptual  processes.  The  experiment  was  repeated  by 
several  of  the  subjects,  but  at  first  all  united  in  affirming  no 
such  change.  The  suggestion  made,  however,  to  one  in- 
dividual gave  the  effect.  It  is  impossible  to  tell  from  James' 
remark  whether  the  observation  is  original  with  him  or  whether 
it  is  quoted. 

To  discuss  the  apparent  size  of  the  after-image  seen  when 
the  eyes  are  closed  leads  one  into  all  the  difficulties  that  have 
followed  the  discussion  of  the  moon's  size.  It  is  interesting 
to  note,  however,  that  when  asked  the  size  of  the  image  result- 
ing from  a  cross  with  arms  one  decimeter  square  as  viewed  at  a 
distance   of   three  meters  the  average   of  the  sizes  noted   by 


44  5.    /.  FRANZ. 

observers  was  7.5  cm.  and  the  image  was  said  to  be  about 
1.87  m.  from  the  eyes.  The  variation  is  large  and  the  individ- 
ual answers  show  characteristics  of  the  several  observers.  The 
subject  might  repay  fuller  and  more  extended  observation. 

Section  8.  The  Retinal  Transfer  of  the  After- 
image.— One  of  the  questions  of  considerable  interest  raised  by 
Newton  in  his  letter  to  Locke  was  that  regarding  the  transfer  of 
an  image  from  the  stimulated  to  the  unstimulated  eye.  "Though 
I  looked  with  my  right  eye  only"  (at  the  sun),  he  says,  "  and 
not  with  my  left,  yet  my  fancy  began  to  make  an  impression 
upon  my  left  eye  as  well  as  upon  my  right.  *  *  *  With  my  left 
eye  I  could  see  the  spectrum  or  the  sun  almost  as  plain  as  with 
my  right." 

Brewster1  independently  noticed  the  phenomenon,  and  it  has 
been  discussed  fully  by  many  observers,  notably  by  Helmholtz, 
Fechner,  Charpentier  and  Titchener.  A  complete  discussion  of 
previous  views  will  be  found  in  Titchener's2  article,  and  only  a 
brief  resume  of  the  general  views  need  here  be  given. 

It  will  readily  be  seen  that  the  transfer  may  be  explained  in 
any  of  the  following  ways  :  (1)  The  appearance  is  an  extension 
of  the  well-known  phenomena  of  binocular  contrast.  (2)  There 
is  a  functional  connection  between  the  retinas,  whereby  one  is 
affected  by  what  affects  the  other.  (3)  The  after-image  has  its 
seat  in  the  brain,  and  not  in  the  retina.  (4)  The  transfer  is 
only  apparent. 

Fechner 3  and  Helmholtz i  are  the  sponsors  of  the  first  theory. 
The  former  stimulated  one  eye  with  a  bright-colored  light,  and 
the  other  was  darkened  or  stimulated  with  a  very  weak  gray 
light.  According  to  him,  the  unstimulated  eye  saw  the  con- 
trasted color  during  the  continuation  of  the  stimulus,  and  this 
left  its  effect  (a  true  after-image,  apparently)  in  this  eye.  In 
other  words,  the  after-image  was  not  transferred,  but  the  op- 

1  Article  on  Accidental  Colours,  in  Edinburgh  Encyclopedia. 
2E.  B.  Titchener,  Ueber  binoculare  Wirkungen  monocular  Reize;  Philos. 
Stud.,  VIII.,  231-310,   1S93. 

3  Fechner,  G.  Th.,  Ueber  einige  Verhaltnisse  des  binocularen  Sehens  ;  Abh. 
d.  k.  Sack.  Ges.  d.  Wi'ss.,  VII.,  481,  i860. 

4  Op.  cit. 


ON  AFTER-IMAGES.  45 

posite  nerve-fibers  of  the  unstimulated  eye  were  excited  during 
the  progress  of  the  sensation,  whence  an  after-image  was  pro- 
duced. This  explanation,  accordingly,  is  only  a  different  form 
of  the  next  theory. 

In  support  of  the  co-excitation  hypothesis  many  other 
physiological  phenomena  may  be  cited.  The  most  evident  of 
these  are  the  facts  that  both  eyes  are  used  as  a  single  instru- 
ment ;  that  they  are  moved  together ;  and  that  if  one  eye  is  in 
the  dark  while  the  other  is  in  light,  the  iris  of  the  former  will 
expand  as  the  iris  of  the  latter  does.  Similar  other  points  indi- 
cate that  the  two  eyes  are  controlled  as  one.  Charpentier1  is  the 
main  exponent  of  this  theory ;  but  his  experiments  upon  the 
varied  sensitiveness  of  the  two  eyes  do  not  seem  to  have  been 
well  chosen  or  conclusive. 

Parinaud,2  Ebbinghaus 3  and  Binet 4  are  supporters  of  the  third 
explanation.  Almost  the  same  experiments  that  convinced 
Fechner  and  Charpentier  were  used  by  them  in  making  the 
new  hypothesis.  It  is  taken  for  granted  that  the  image  is  seen 
by  the  eye  that  is  open,  and  to  them  the  only  satisfactory  ex- 
planation is  that  the  image  lies  in  the  cerebral  center  of  vision, 
and  not  in  the  retina. 

While  accepting  the  same  general  point  of  view,  Titchener 
introduces  a  new  series  of  experiments  to  prove  that  the  transfer 
is  real,  and  is  not  an  apparent  one.  The  experiments  included 
observations  upon(  i  )the  relative  duration  in  the  stimulated  and  in 
the  non-stimulated  eye  ;  (2)  the  variations  in  the  fluctuations  ;  (3) 
the  color-changes,  and  (4)  the  differences  in  brightness.  The 
results  of  these  experiments  by  Titchener  are  summed  up  as  fol- 
lows :  (1)  The  image  in  the  non-stimulated  eye  lasts  a  shorter 
time  than  in  the  stimulated  eye.  (2)  The  relative  time  which 
the  two  phases  of  the  secondary  (i.  e.,  in  the  non-stimulated 
eye)  image  take  is  in  no  way  similar  in  the  primary  eye. 
(3)   The  phenomenon  occurs  also  under  conditions  when  a  mix- 

1  Charpentier,  A.,  La  lumiere  et  les  couleurs,  Paris,  188S.  Sur  les  connex- 
ions functionnelles  des  deux  retines.  C.  Rend,  de  la  Soc.  de  Biol.,  8  Ser.,  II., 
364,  1885. 

2  C.  Rend,  de  la  Soc.  de  Biol.,  13  May,  1S82. 

3  Archiv  f.  die  gesammle  Physiol.,  XLVI.,   498,  1S91. 

4 La  Psychologie  du  raisonnement.     Paris,  1886,  p.  45  ft. 


4°  5.    /.   FRANZ. 

ture  of  both  fields  of  vision  is  made  impossible.  (4)  There  is 
a  constant  difference  in  brightness  between  the  negative  comple- 
mentary images  in  the  two  eyes.  (5)  Experiments  upon  a 
monocularly  color-blind  person  gave  evidence  of  the  second 
image. 

On  the  other  side  of  the  question  we  find  Delabarre.  To 
him  the  secondary  after-image  is  an  illusion ;  the  transfer  is  only 
apparent.  Considering  the  experiments  made  by  Parinaud  and 
Binet,  he  says:  "A  serious  difficulty  in  settling  the  question 
lies  in  the  well-known  impossibility  of  separating  the  visual 
fields  of  the  two  eyes.  Whether  one  eye  or  both  are  open ; 
whether  they  are  focussed  on  the  same  point  or  are  held  par- 
allel, or  squinted,  or  even  jammed  into  all  sorts  of  relative  posi- 
tions by  fingers  inserted  into  their  sockets,  the  field  of  each  will 
appear  to  coincide  with  the  field  of  the  corresponding  portion  of 
the  retina  of  the  other.  If  an  after-image  be  found  in  both  to- 
gether, one  image  only  will  be  seen  whatever  their  relative 
positions  ;  and  if  the  image  be  found  on  one  alone,  it  will  yet  be 
seen  in  the  corresponding  portion  of  the  field  of  the  other,  pro- 
vided that  the  brilliancy  of  the  second  field  be  not  so  great  as  to 
obscure  the  much  weaker  sensation  of  the  image.  In  reality, 
in  this  experiment  the  after-image  never  does  appear  on  the  left 
field  until  the  last  eye  has  so  greatly  darkened  as  to  allow  it  to 
be  seen ;  and  in  the  periodical  increases  in  brilliancy  of  the  left 
field  the  image  disappears.  It  will  thus  be  seen  that  a  retinal 
seat  of  the  after-image  explains  all  the  facts  as  easily  as  does  a 
central  seat.  Hence  the  assumption  of  the  latter  by  M.  Binet 
and  others  is  entirely  superfluous  *  *  *  noi  merely  super- 
fluous, but  impossible."1 

The  results  of  five  experiments  tend  to  strengthen  this  posi- 
tion :  (1)  There  is  a  difference  in  brightness  of  the  image 
when  the  stimulated  eye  is  open  or  shut.  (2)  With  an  image  in 
the  right  eye,  open  it  and  the  image  is  clearly  seen  ;  open  the  left 
eye  with  the  right  closed,  and  the  image  is  blotted  out.  Such 
an  effect  is  probably  due  to  binocular  mixture.  (3)  With  both 
eyes   open,  the  image   being   in  the  right,  place   a  pencil   or 

1  Delabarre,  E.  B.,  On  the  Seat  of  Optical  After-images  ;  Amer.  Jour. 
Psych.,  II.,  326-328.    1889. 


ON  AFTER-IMAGES.  47 

similar  object  before  the  left ;  no  change  in  the  image  occurs. 
Place  an  obstruction  before  the  right,  and  the  image  disappears 
temporarily  or  permanently.  (4)  Obtain  a  strong  image  with 
the  right  eye  ;  if  projected  on  a  white  surface,  it  appears  light 
green.  If  both  eyes  are  covered  tightly,  so  that  no  light  gets  in, 
the  image  is  dark  green.  If  the  eyes  are  closed,  but  not 
covered,  and  light  gets  in,  the  image  is  rose-colored.  Now 
"obtain  an  image  with  the  right  eye,  then  close  and  cover  it 
and  open  the  left  eye.  Now  the  left  field  darkens  and  the  dark- 
green  after-image  appears ;  this  gives  place  to  the  rose-colored 
image  if  the  covering  is  removed  from  the  right  eye,  and  the 
eye  kept  closed ;  and  this  in  turn  to  the  light  green  if  the  right 
eye  is  opened.  These  three  colors  can  be  made  to  succeed 
one  another  indefinitely  without  in  any  way  interfering  with  the 
open  left  eye,  which,  alone,  according  to  M.  Binet,  is  the 
source  of  all  the  visual  impressions  present."  (5)  In  retinal 
rivalry,  on  obtaining  an  image  with  the  right  eye,  look  with 
both  eyes  through  colored  glasses  at  a  background.  When  the 
color  seen  with  the  right  eye  is  predominant  the  image  is  seen ; 
when  the  color  seen  with  the  left  eye  is  predominant  no  image  is 
apparent. 

Professor  Titchener's  experiments,  like  Binet's,:may  also  be 
explained  by  considering  the  image  a  peripheral  as  well  as  a 
central  event.  The  variation  in  brightness  is  explained  from 
the  peripheral  point  of  view,  to  my  satisfaction,  by  Delabarre's 
considerations.  The  apparent  difference  in  the  duration  of  the 
image  is  readily  understood  when  one  learns  that  pressure  of  any 
sort  on  the  eye  will  disturb  the  image.  The  mere  closing  of 
the  eyelid  would  suffice  for  this.  When  Titchener  speaks  of 
the  occurrence  of  the  phenomena  even  under  '  conditions  when 
a  mixture  of  both  fields  is  impossible,'  he  seems  to  forget  that 
the  two  eyes  always  act  as  one,  and  that  it  is  impossible  to  tell 
(by  ordinary  inspection)  in  which  eye  a  sensation  is.  One  of 
the  subjects  used  in  the  present  research,  a  man  who  had 
worked  during  two  years  upon  binocular  rivalry,  advises  me  that 
he  is  now  unable,  even  after  such  great  practice,  to  tell  in 
which  eye  the  sensation  is.  It  seems  probable,  in  view  of  other 
experiments  noted  above,  that  Dr.  Titchener's  subjects  were  in- 


4*5  5.   /.   FRANZ. 

fluenced  to  a  great  extent  by  suggestion.  It  has  been  seen  what 
a  great  part  this  plays  in  the  duration  and  in  the  spatial  charac- 
ter of  the  after-image ;  and  it  is  not  unlikely  that  a  similar  in- 
fluence is  felt  when  looking  for  an  image  with  an  unstimulated 
eye. 

To  my  mind,  the  only  experimental  results  that  might  indi- 
cate the  central  seat  of  the  after-image  are  the  well-known 
experiments  with  suggested  after-images.  Binet  gives  the 
following  account  of  the  experiments  •}  "  Wundt  has  shown 
that  the  simple  image  of  a  color,  imagined  for  a  long  time, 
gives  rise  to  an  after-sensation  of  the  complementary  color.  * 
If  one  mentally  looks  fixedly  for  some  time  at  a  red  image,  he 
perceives  a  green  tint  upon  opening  the  eyes  and  looking  to- 
ward a  white  surface.  This  experiment  is  difficult  to  repeat, 
for  it  is  necessary  to  have  a  power  of  visualization  that  not 
every  one  has.  To  take  me  as  an  example,  I  am  not  even  able 
to  imagine  a  color  clearly.  I  am  a  very  mediocre  visualizer, 
so  it  is  not  surprising  that  I  cannot  obtain  the  colored  after- 
image. But  my  friend  Dr.  F£re  does  this  readily.  He  can 
represent  to  himself  a  red  cross  sufficiently  intense  to  see  on  a 
piece  of  paper  a  green  cross  following  it ;  moreover,  he  sees  not 
only  the  color  but  the  shape."  A  similar  experiment  has  been 
made  with  hypnotized  subjects.  A  red  cross  is  suggested  and 
then  suggested  away,  and  the  green  after-image  noted.  In  the 
case  mentioned  by  Binet,  however,  it  seems  not  to  have  occurred 
to  him  that  possibly  the  resultant  after-image  was  suggested 
just  as  was  the  original  red  cross.  The  experiment  has  not 
been  made  in  a  rigidly  scientific  manner,  and  is  inadequate.  I 
have  been  unable  to  get  a  sufficiently  good  visualizer  who  was 
at  the  same  time  na'ive  regarding  after-images,  and  I  have  not 
been  able  to  repeat  the  experiment  with  due  precautions. 

The  case  with  a  full  hypnotized  subject  seems  not  well  au- 
thenticated. However  if  the  question  of  its  authenticity  be 
waived  for  a  moment,  the  question  immediately  arises,  "  Is  the 
after-image  the  result  of  seeing  the  imaginary  red  cross,  or  is  it 
not  in  this  case  also  a  new  suggestion?"  The  precaution  not  to 
give  such  a  suggestion  seems  not  to  have  been  taken,  and  this 

1  La  Psychologie  du  Raisonnement,  Paris,  1886,  p.  41. 


ON  AFTER-IMAGES.  49 

would  vitiate  the  experiment.  Even  if  this  precaution  were 
taken  and  the  same  result  found,  the  central  hypothesis  would 
not  be  proven.  Other  experiments  with  hypnotized  subjects 
would  seem  to  indicate  that  from  suggestion  certain  tissues 
may  be  modified  so  as  to  leave  thereafter  a  noticeable  ef- 
fect. This  is  true  of  the  experiments  in  which  a  blister  is 
produced  by  suggesting  the  application  of  some  known  medium 
which  ordinarily  will  produce  such  an  effect.  With  such  a 
case  explained  upon  purely  peripheral  grounds,  would  it  not 
seem  probable  that  the  eye  was  also  so  stimulated  when  the  red 
cross  was  imagined? 

Delabarre  suggests  the  following  experiment,  a  trial  of  which 
might  aid  in  giving  some  negative  testimony.  Hypnotize  a  sub- 
ject and  suggest  a  sensation  that  he  may  get  a  good  after- 
image. Then  by  suggestion  paralyze  the  sight  of  this  eye.  If 
no  after-image  is  seen  in  the  other  eye,  it  will  indicate  that  there  is 
no  transfer ;  and  if  an  image  is  presumably  seen,  "  it  will  merely 
indicate  that  the  paralysis  of  the  right  optic  nerve  has  not  been 
complete."1  Delabarre's  radical  statement  should  have  its  will 
toned  to  may.  The  result  in  either  case  would  not  decide 
whether  the  image  was  central  or  peripherally  transferred. 

An  experiment  which  may  indicate  a  line  for  investigation 
has  been  tried  by  me.  I  attempted  to  stimulate  only  the  optic 
nerve  and  the  cerebral  visual  centers  to  discover  what  effect,  if 
any,  was  left  by  such  stimulation.  Electrodes  were  placed  upon 
the  head,  and  when  the  circuit  was  closed  and  broken  a  vivid 
flash  of  light  was  produced.  All  the  experiments  gave  nega- 
tive results.  No  after-image  was  produced.  It  would  be  sup- 
posed that,  if  the  image  was  of  central  origin,  the  stimulation 
of  the  central  centers  would  give  such  an  after-image.  In  this 
experiment  it  is  not  certain  that  the  retina  was  not  stimulated,  but 
that  the  brain  center  was  (either  directly  by  the  electricity  or  by 
the  retina,  which  was  in  turn  stimulated  by  the  electric  current). 
It  would  seem  as  if  an  after-image  should  have  been  produced 
in  either  case  if  the  seat  was  cerebral. 

Another  experiment,  which  gives  only  a  partial  and  nega- 
tive answer  to  the  question  of  transfer,  has  been  made  by  me. 

^p.  cit. 


50  5.    /.   FRANZ. 

That  portion  of  the  right  eye  which  corresponds  to  the  blind  spot 
of  the  left  eye  was  stimulated  to  see  whether  there  would  be  an 
apparent  transfer.  If  a  transfer  (apparent)  occurred,  it  could  not 
be  accepted  as  real,  since  with  the  corresponding  portion  of  the 
left  eye  we  can  see  nothing.  If  no  image  is  apparent  on  open- 
ing the  left  eye,  we  learn  nothing  new,  and  neither  theory  is 
benefited.  The  fact  is,  an  image  appeared  upon  opening  the 
left  eye,  and  we  are  left  to  conclude  that  the  transfer  in  this  case 
is  apparent  and  in  other  cases  it  is  likely  to  be  apparent. 

Such  an  experiment  is  of  value  in  that  it  is  a  link  in  the 
evidence  tending  to  exclude  one  hypothesis.  The  image  seems 
not  to  be  transferred,  but  is  either  central  or  peripheral.  The 
central  situation  seems  to  me  improbable  in  view  of  the  results 
of  Delabarre's  expei-iments  and  of  the  experiment  on  brain 
stimulation.  The  changes  in  the  image  under  objective  changes 
in  the  organ  originally  stimulated  indicate  a  peripheral  seat  only 
in  the  stimulated  eye ;  and  the  interference  with  the  stimulated 
eye  interfering  with  the  progress  of  the  image  indicates  that 
the  after-image  is  not  transferred. 


ON  AFTER-IMAGES.  51 


Part  II.     Historical  and  Descriptive. 

Section  9.  Relation  of  After-images  to  Sensation, 
to  Imagination  and  to  Memory. — The  intimate  relation  of 
after-images  to  sensation,  to  memory  and  to  imagination  makes 
the  phenomena  of  great  psychological  interest.  Seeming  to  be 
the  connecting  link  between  sensation  and  the  idea,  the  study 
of  this  relation  is  of  considerable  epistemological  importance. 

The  differences  between  visual  after-images  and  the  images 
of  the  imagination  have  been  thoroughly  discussed  by  Fechner.1 
His  conclusions  are  summarized  by  James  as  follows  :2 

"After-images  first  coercive  ;  seem  unsubstantial,  vaporous  ;  are  sharp  in 
outline  ;  are  bright ;  are  almost  colorless  ;  are  continuously  enduring  ;  cannot  be 
voluntarily  changed  ;  are  exact  copies  of  the  originals  ;  are  more  easily  got  with 
shut  than  with  open  eyes  ;  seem  to  move  when  the  head  or  eyes  move  ;  the  field 
within  which  they  appear  (with  eyes  covered)  is  dark,  contracted,  flat,  close  to 
the  eyes  and  the  images  have  no  perspective  ;  the  attention  seems  directed 
forward  toward  the  sense  organ  in  observing  after-images. "  On  the  other  hand, 
"  imagination-imagesizA  subject  to  our  spontaneity;  have,  as  it  were,  more 
body  ;  are  blurred  ;  are  darker  than  the  darkest  black  of  the  after-images  ;  have 
lively  coloration  ;  incessantly  disappear  and  have  to  be  renewed  by  an  effort 
of  the  will  (at  last  even  this  fails  to  revive  them);  can  be  exchanged  at  will 
for  others  ;  cannot  violate  the  necessary  laws  of  appearance  of  their  originals  ; 
e.  g.,  a  man  cannot  be  imagined  from  in  front  and  behind  at  once  ;  the  imagi- 
nation must  walk  around  him,  so  to  speak  ;  are  more  easily  had  with  open  than 
with  shut  eyes  ;  need  not  follow  the  movements  of  head  or  eyes  ;  the  field  is  ex- 
tensive in  three  dimensions,  and  objects  can  be  imagined  in  it  above  or  behind 
almost  as  easily  as  in  front ;  in  imagining,  the  attention  feels  as  if  drawn  back- 
wards towards  the  brain.  Finally  Fechner  speaks  of  the  impossibility  of 
attending  to  both  after-images  and  imagination-images  at  once,  even  when  they 
are  of  the  same  object  and  might  be  expected  to  combine." 

The  above  account  is  true  only  of  Fechner  himself ;  he  re- 
marks that  results  from  other  individuals  show  certain  character- 
istic differences.  Dr.  Lay  finds  that  he  agrees  with  Fechner 
only  in  the  following  particulars  :  the  after-images  are  coercive, 
cannot  be  voluntarily  changed,  seem  to  move  with  the  eyes,  and 
the  attention  is  directed  forward  toward  the  sense  organ, 
while   the    images    of    the   imagination    are    directly    opposite 

1  Elemente  der  Psychophysik,  II.,  46S  ff. 

2  Principles  of  Psychology,  Vol.  II.,  50. 


52  S.   L   FRANZ. 

in  these  particulars.1  Regarding  the  other  differences,  it  is  not 
clear  whether  Dr.  Lay  disagrees  with  Fechner  regarding  the 
characterization  of  the  after-image  or  of  the  images  of  imagi- 
nation. 

From  the  numerous  observations  of  my  subjects  and  myself 
any  one  of  these  qualifications  of  the  after-image  may  be  contra- 
dicted. The  fact  that  we  do  not  see  an  after-image  after  every 
visual  sensation,  the  fact  that  it  requires  an  amount  of  attention 
to  perceive  it,  would  indicate  that  the  phenomenon  is  not  so 
coercive  as  Fechner  believed.  The  after-images  are  not  color- 
less ;  sometimes  they  are  colored  more  lively  than  ordinary  * 
sensations,  and  often  their  color  is  more  intense  than  that  of  the 
imagination-images.  The  images  are  not  continuously  endur- 
ing, but  have  many  fluctuations.  We  have  already  seen  (Part 
i,  Sec.  7)  that  the  after-image  often  has  the  appearance  of 
solidity.  The  probable  reason  for  the  usual  two-dimensional 
character  lies  in  the  fact  that  nearly  all  objects  bright  enough 
to  produce  after-images  are  of  two  dimensions  only.  The 
colored  paper,  the  window  panes,  the  gas  flame,  etc.,  are 
plane  surfaces  and  in  the  sensation  the  effect  of  solidity  is  not 
gotten  directly  from  these,  but  rather  from  their  surroundings, 
which  usually  have  not  the  requisite  intensity  for  the  production 
of  after-images.  To  Fechner's  differences  it  may  be  added  that 
the  after-image  is  sharp  and  clear  only  if  very  near  the  point  of 
fixation,  while  the  imagination-image  may  be  a  clear  represen- 
tation of  a  scene  all  of  which  could  not  be  noted  by  the  eye  at 
one  time.  James  adds  as  a  universal  proposition  that  the  after- 
images seem  larger  if  we  project  them  on  a  distant  screen,  and 
smaller  if  we  project  them  on  a  near  one,  while  no  such  change 
takes  place  in  mental  pictures.2 

The  name  of  memory-after-image  is  given  by  Fechner  to 
the  instantaneous  positive  effects  of  sensation.3  These  images 
are  distinguished  from  ordinary  after-images  by  the  following 
characteristics  :     (i)  Their  originals  must  have  been  attended  to 

1  W.  Lay,  Mental  Imagery,  p.  2.  Monograph  Supplement  No.  7  to  The  Psy- 
cJiological  Jieview,  1898. 

2  Principles  of  Psychology,  p.  51,  note. 
3Elemente  der  Psychophysik,  II,  p.  491  ff. 


ON  AFTER-IMAGES.  53 

only  such  parts  of  the  compared  originals  as  have  been  attended 
to  appearing :  this  is  not  the  case  in  common  visual  after- 
images. (2)  The  strain  of  attention  toward  them  is  inward,  as 
in  ordinary  remembering ;  not  outward,  as  in  observing  an  or- 
dinary after-image.  (3)  A  short  fixation  of  the  original  is  better 
for  the  memory-after-image ;  a  long  one  for  the  ordinary  after- 
image. (4)  The  colors  of  the  memory-after-image  are  never 
complementary  of  those  of  the  original.  It  is  difficult  to  state 
the  relation  between  the  memory-after-image  and  the  memory- 
image,  but  the  two  seem  almost  equally  different  from  the  true 
after-image.  Many  memory-after-images  have  an  overpower- 
ingly  coercive  quality.  Examples  of  this  are  numerous.  A 
revolting  scene  will  often  leave  a  lasting  memory-image.  The 
sight  of  some  one  drowning  or  a  railway  accident  will  remain 
in  the  mind  for  weeks ;  the  details  will  appear  in  consciousness 
unexpectedly,  and  will  overpower  us  as  did  the  originals.  But 
the  effect  is  not  produced  so  often  with  the  visual  as  with  the 
auditory  memory-after-images.  The  snatches  of  melody  that 
one  hears  continually  in  the  mind's  ear  after  a  concert,  or  the 
popular  tunes  that  often  rise  in  consciousness  and  compel  us  to 
hum  them  over  and  over  again,  have  a  coercive  quality  un- 
paralleled in  the  phenomena  of  after-images. 

In  many  ways  the  after-images  can  be  considered  a  true  sen- 
sation. One  of  the  chief  sensational  conditions  is  present,  viz., 
a  change  in  the  sense  organ ;  and  many  of  the  more  mental 
conditions  of  the  sensation  are  equalled.  At  times  it  is  impossi- 
ble to  say  whether  it  is  an  after-image  or  a  sensation  that  is  in 
consciousness.  A  friend  relates  that  his  child,  who  had  looked 
at  the  sun  and  then  turned  around,  tried  to  point  out  to  a  sister 
the  second  sun  (7.  e.,  the  after-image,  which  was  real  to  him). 
Older  people  sometimes  make  the  same  mistake,  and  correct  the 
error  only  from  later  observations.  Such  an  uncertainty  is  due 
primarily  to  a  wonderful  clearness  of  outline,  brightness  of  color 
and  great  intensity  of  the  after-image.  To  these  differences 
Sully1  adds  "  definiteness  of  localization  (either  in  the  field  of 
objects  if  the  eyes  are  open,  or  in  the  dark  field  if  they  are 
shut)."     Such  conditions  are  not  often   produced  in  one's  daily 

iThe  Human  Mind,  I.,  p.  17S. 


54  S.    /.  FRANZ. 

life,  but  may  be  obtained  by  suitable  experiment.  On  the  other 
hand,  the  moving  of  the  after-image  with  the  eye,  its  fluctua- 
tions, its  negative  quality,  its  (usually)  plane  character,  and  the 
fact  that  it  is  not  doubled  by  lateral  pressure  upon  the  eyeball, 
differentiate  the  after-image  from  a  sensation. 

Section  io.  History.1 — The  phenomena  variously  known 
as  after-images,  recurrent-images  and  ocular  spectra  (Germ. 
nachbilder ;  Fr.  couleurs  accidentelles,  persistence  des  im- 
pressions') seem  to  have  been  noted  first  by  Aristotle.  He 
compared  the  images  of  the  dream  state  to  them,  and  speaks  of 
them  as  if  they  were  familar  to  his  audience.  The  account  is 
as  follows : 

"  It  is  evident  that  when  we  look  at  anything  for  a  long  time,  and  then 
turn  away  our  eyes,  the  sensation  continues  ;  just,  for  example,  as  when  we 
look  at  something  dark  after  having  looked  at  the  sun,  it  happens  that  on  ac- 
count of  the  force  from  the  light  still  remaining  in  our  eyes  we  see  nothing 
(*'.  e.,  of  the  shaded  object).  And  if,  after  having  looked  at  a  color  for  a  long 
time  we  turn  away  our  eyes,  this  same  thing  happens  ;  and  if  we  should  turn 
away  our  eyes  after  having  looked  at  the  sun  or  some  other  bright  object,  it 
happens  that  the  eye  sees  first  the  same  color,  this  color  then  changes  to  red, 
then  to  purple,  and  after  becoming  black  disappears."  2 

Later  the  after-image  is  mentioned  by  St.  Augustine,  and 
by  the  Arab  Alhazin,  who  was  probably  attracted  to  their  study 
from  reading  Aristotle's  works. 

In  the  seventeenth  century  (according  to  Helmholtz,  in 
1634)  Peiresc  "observed  1000  times  that  when  he  had  looked 
upon  the  window  distinguished  with  wooden  bars  and  squares 
of  paper,  he  carried  the  form  thereof  some  time  after  in  his 
eyes ;  but  with  this  difference,  that  if  he  kept  his  eyes  shut,  he 
seemed  to  behold  the  bars  dark,  and  the  paper  squares  white, 
as  he  had  at  first  seen  them ;  but  if  he  looked  with  his  eyes 
upon  a  dark  wall,  then  the  paper  squares  seemed  dark,  and  the 
bars  of  the  same  whiteness  with  the  wall."3 

Kircher,  Mariotte,  Boyle,  Fabri,  in  the  succeeding  century 

1  A  short  historical  resume  will  be  found  in  Helmholtz,    Physiol.    Optik, 
2  Aufl.,  pp.  836-837. 
sDe  Somniis. 
3Life  of  Peiresc,  London,  1657,  Book  IV.,  p.  101. 


ON  AFTER-IMAGES.  55 

made  slight  additions  to  our  knowledge  regarding  the  appear- 
ances. Newton's  account  of  his  memorable  after-image  (see 
Part  I.,  Sec.  4,  p.  37)  was  sent  to  Locke  about  this  time.  In  it 
he  seems  to  be  the  first  after  Aristotle  to  call  attention  to  their 
intimate  relation  to  the  more  (so-called)  mental  images. 

Buff  on1  in  1743  related  his  experience  with  after-images 
projected  on  differently  colored  backgrounds.  He  found  that 
the  image  fused  with  the  background,  and  formed  a  color  which 
was  a  combination  of  the  true  color  of  the  after-image  and 
that  of  the  background.  On  account  of  the  variety  of  the 
phenomena  and  because  he  was  unable  to  account  for  all  the 
appearances,  Buff  on  called  the  after-images  '  couleurs  acci- 
dentelles.'  These  experiences  of  Buffon  were  later  confirmed 
by  Gergonne,  who  made  a  number  of  new  experiments  similar 
in  character. 

The  first  theory  after  Aristotle's  (^continuation  of  the 
stimulus)  was  that  of  Jurin  (1758?).  Apparently  considering 
only  the  negative  phases,  he  regarded  the  after-image  as  due 
to  a  process  in  the  retina  the  reverse  of  what  went  on  in  normal 
sensation.  In  expression  this  view  is  strikingly  like  some  most 
recent  ones,  although  Jurin  had  not  the  same  ideas  as  the  more 
modern  writers. 

Scherffer3(i76i),  noting  the  negative  images  on  a  light  back- 
ground, supposed  the  phenomena  to  be  caused  by  a  temporary 
loss  of  sensibility  of  the  retina  for  one  color.  He  thought  that 
the  eye,  having  undergone  a  prolonged  action  from  rays  of  a 
certain  color,  lost  momentarily  its  sensibility  for  a  weaker 
stimulus  of  rays  of  that  color.  Thus,  on  looking  at  white  the 
eye  is  stimulated  by  rays  of  light  of  different  color — red,  green, 
blue,  etc.,  and  the  retina  recombines  these  into  white.  Now, 
after  having  looked  for  a  time  at  red,  the  retina  becomes  fa- 
tigued for  rays  of  that  color,  and  when  a  white  object  is  then 
fixated  the  various  rays  composing  white  are  seen  with  the 
exception    of   the    red,  thus  producing  a  bluish-green  image. 

1Memoires  de  l'Acad.  des  Sciences  de  Paris,  1743,  p.  213. 
"Essay  on  Distinct  and  Indistinct  Vision,  p.  170  ff,  of  Smith's  Optics,  Cam- 
bridge, 1738. 

3  Dissertation  sur  les  couleurs  accidentelles  ;  Jour,  de  Physique,  XXVI.,  1785. 


56  5.   /.   FRANZ. 

Plateau1  conclusively  shows  that  this  theory  is  inadequate  in  that 
it  does  not  account  for  the  negative  after-images  which  are  seen 
perfectly  in  most  complete  darkness.  A  later  theory  proposed 
by  Scherffer  was  that  the  after-image  is  due  to  a  prolongation 
of  a  feebler  stimulation  produced  by  rays  different  from  the 
dominant  color.  For  example,  in  looking  at  a  red  square  we 
see  not  only  red  light,  but  also  some  blue  and  green  light,  and 
when  the  red  is  taken  away  the  blue  and  green,  which  have 
not  been  too  intense  to  overpower  the  eye,  continue  to  be  seen, 
thus  producing  the  after-image.  Were  this  true,  it  would  be  an 
example  of  a  lesser  light  having  a  greater  effect  than  one  of 
great  intensity. 

De  Godart2  is  responsible  for  two  (so-called)  theories.  The 
first  of  these  is  a  very  fanciful  one.  Arranging  the  colors 
like  the  tones  in  a  musical  scale  (black,  blue,  green,  red,  yellow, 
white),  he  believed  that  the  direct  continuation  of  a  sensation  was 
as  much  lower  than  white  as  the  sensation  was  higher  than 
black.  A  sympathetic  action  was  set  up  in  the  retina  just  as 
sympathetic  tones  are  noted  on  a  musical  instrument.  "  Such  a 
theory,"  Plateau  remarks,  "scarcely  needs  refutation."  The 
other  theory  of  De  Godart  is  as  follows : 

Voici  une  autre  thdorie  de  ces  phdnomenes :  c'est  de  dire  tout  uniment  qu'une 
fibre  dbranlee  par  un  objet  reste  incapable  de  dormer  la  sensation  d'un  autre, 
aussi  longtemps  qu'elle  conserver  l'impression  du  premier,  et  que  les  diffdrentes 
couleurs  dtant  experimdes  par  des  portion  d'une  meme  fibre  d'autant  plus, 
courtes  que  le  ton  de  la  couleur  est  plus  vif,  c'est  la  partie  quin,  a  pas joud  qui 
excitde  par  le  blanc  a  le  faire,  donne  la  couleur  accidentelle.3 

This  seems  to  be  only  a  badly  conceived,  a  poorly  expressed 
theory  of  insensibility  like  that  proposed  by  Schaeff er ;  and 
coming,  as  it  does,  without  elaboration  toward  the  close  of  his 
paper,  it  seems  to  indicate  that  the  hypothesis  is  not  well  con- 
sidered by  the  author. 

Darwin*  (R.  W.),  having  considered  all  the  known  facts, 

1Essai  d'une  Theorie  generate  comprenant  l'ensemble  des  apparences  visuel- 
les  qui  succedent  a  la  contemplation  des  objets  coleres.  *  *  *  la  persistance 
des  impressions  de  la  Retine,  les  couleurs  accidentelles,  etc.  ;  Ann.  de  Chimie  et 
de  Physique,  LVIII.,  337-406,  1835. 

''■Jour-  de  Physique,  VIII.,  1776. 

sS5id. 

4R.  W.  Darwin,  New  Experiments  on  OcularjjSpectra  of  Light  and  Colours; 
Philos.  Trans.,  LXXVI.,  313-348,  17S6.  Also  found  in  E.  Darwin's  Zoonomia, 
4  Am.  Edit.,  1818,  Vol.  I.,  p.  443-466.     See  also  Vol.  I.,  p.  10  ff. 


ON  AFTER-IMAGES.  57 

attempted  to  group  them  into  four  classes:  (i)  Images  owing 
to  a  less  sensibility  of  a  defined  part  of  the  retina.  (2)  Images 
owing  to  a  greater  sensibility  of  a  defined  part  of  the  retina. 
(3)  Images  that  resemble  their  object  in  color  as  well  as 
form.  (4)  Images  that  are  of  a  color  contrary  to  that 
of  their  object.  From  the  consideration  of  these  different 
facts,  he  was  led  to  believe  that  a  part  of  the  retina  became  fa- 
tigued by  a  color  and  became  insensible  to  rays  of  that  color, 
and  that  this  part  of  the  retina  then  took  up  a  mode  of  action 
opposite  to  that  which  produced  the  sensation.  The  details  of 
this  theory  are  interesting  in  view  of  what  is  at  present  known  of 
the  retina's  action.  He  says  "the  effect  of  the  activity  of  the 
retina  may  be  to  alter  its  thickness  or  thinness,  so  as  better  to 
reflect  or  transmit  the  colours  which  stimulate  it  into  action."1 
Possibly  "the  muscular  actions  of  the  retina  constitute  the 
sensation  of  lights  and  colours ;  and  the  voluntary  repetitions  of 
them,  when  the  object  is  withdrawn,  constitute  our  memory  of 
them."2 

Contrast  was  used  as  an  explanation  by  C.  A.  Prieur  (or 
Prieur  de  la  Cote  d'Or.  [Fechner]  ) .  Numerous  observations 
were  made  in  the  succeeding  years,  but  they  were  only  slight 
variations  from  the  previous  work.     They  are  unimportant. 

In  1835,  Brewster3  discussed  the  various  color-changes,  and 
concluded  that  the  primary  color  and  the  color  of  the  after-image 
existed  in  the  retina  simultaneously,  in  the  same  manner  as  a 
fundamental  tone  and  its  harmonic.  After  the  primary  light 
has  ceased  the  color  of  the  after-image  continues.  This  is  al- 
most identical  with  the  first  theory  of  de  Godard.  In  construct- 
ing his  theory  Brewster  seems  to  rely  greatly  upon  the  various 
phenomena  of  simultaneous  contrasts,  considering  these  as  repre- 
sentatives of  the  after-image  process. 

Having  considered  the  inadequacy  of  each  of  the  foregoing 
theories  from  Jurin  to  Brewster,  Plateau4  made   a  careful  ex- 

lP/tilos.  Trans.,  LXXVL,  p.  348. 
*Ibid. 

8  In  Edinburgh  Encyclopedia,  Vol.  I.,  article  Accidental  Colours.  Also 
Philos.  Mag.,  IV.,  354,  1S34. 

*Ann.  de  Chimie  et  de  Physique,  LVIII.,  337-406. 


58  5.   /.   FRANZ. 

amination  of  all  the  known  facts  preparatory  to  constructing  a 
new  theory.  His  theory,  which  follows,  is  partly  a  combination 
of  the  ideas  of  his  predecessors,  particularly  those  of  Jurin  and 
Darwin.  He  says,  "  we  must  conclude  that  the  accidental 
image  results  from  a  particular  modification  of  the  organ,  which 
spontaneously  gives  us  a  new  sensation."  "When  the  retina 
has  undergone  the  action  of  rays  of  a  certain  color,  it  resists 
the  action  of  that  color  and  tends  to  regain  its  normal  state  with 
a  force  more  and  more  intense.  Then  if  the  excitation  is  sud- 
denly removed,  it  returns  to  its  normal  state  by  an  oscillatory 
movement  as  much  more  intense  as  the  action  has  been  pro- 
longed." The  first  primary  image,  which  he  considered  a  pro- 
longation of  the  stimulus,  was  noted  by  him,  and  he  concludes 
"  that  when  the  retina,  after  having  been  excited  for  some  time 
by  the  presence  of  a  colored  object,  is  suddenly  removed  from 
this  excitation,  the  sensation  produced  by  the  object  continues 
to  exist  for  a  very  short  time,  after  which  the  retina  spon- 
taneously takes  on  a  state  opposite  to  the  first,  whence  there 
results  the  sensation  of  the  accidental  color. 

Dove,  Scoresby,  Grove,  Seguin,  Brucke  and  Aubert  noted 
the  after-images  of  moving  objects,  and  the  appearance  and 
reappearance  of  the  images  under  increased  and  decreased  eye 
illumination.  Brucke  and  Aubert  noticed  the  after-images  re- 
sultant from  instantaneous  illumination  by  the  electric  spark. 

Fechner,  who  lost  his  eyesight  mainly  because  of  his  long- 
continued  study  of  after-images  from  very  bright  lights,  pro- 
posed the  theory  which  has  usually  been  associated  with  the 
name  of  Helmholtz,  viz.,  that  the  positive  phase  of  the  image 
is  a  continuation  of  the  stimulus,  and  the  negative  and  comple- 
mentary phases  are  due  to  retinal  exhaustion.  This  hypothesis 
is  the  one  accepted  by  Wundt.  The  theory  is  inadequate 
because  it  does  not  account  for  an  after-image  whose  course 
(fluctuation)  is  as  follows:  Pos.,  neg.,  neg.,  pos.,  pos.,  neg., 
neg.,  neg.,  etc. 

Hering  regards  the  positive  after-image  as  a  continuation  of 
the  stimulus,  and  the  negative  phases  as  reactions  of  the  visual 
elements  to  a  state  of  equilibrium,  the  '  assimilation  or  dissimi- 
lation in  some  of  the  visual  substances.'     The  theory  of  light 


ON  AFTER-IMA  GES.  59 

sensation  proposed  by  Mrs.  Franklin  would  account  for  the  phe- 
nomena in  approximately  the  same  manner.  It  will  be  noticed 
that  both  these  theories  are  in  terms,  though  not  wholly  in  sense, 
the  same  as  Darwin's. 

Acceptingthe  general  theoretical  position  of  the  Young-Helm- 
holtz  hypothesis  of  color  vision,  Bidwell  would  account  for  the 
phenomena  as  due  to  a  '  reaction  of  the  violet  nerve  fibers 
only.'1  Four  reasons  are  given  for  this  view:  "  (i)  With 
white  light  the  recurrent  colour  is  violet.  (2)  In  the  recurrent 
spectrum  of  the  complete  spectrum  no  colour  but  violet  can  be 
detected.  (3)  A  pure  red  light,  however  intense,  gives  no  re- 
current image.  *  *  *  (4)  The  apparently  blue  colour  of  the 
ghost  of  simple  spectrum  yellow  is  just  as  well  produced  by  a 
compound  yellow  consisting  of  green  and  red,  the  latter  of 
which  is  inert  when  tested  separately."  It  should  be  remarked, 
however,  that  some  of  these  observations  have  been  disputed, 
and  there  is  always  danger  of  accepting  a  theory  which  is 
proved  by  experiments  devised  after  the  acceptance  of  the 
theory. 

It  is  to  be  regretted  that  all  these  theories  have  made  little 
addition  to  our  knowledge  regarding  the  phenomena.  Except 
the  work  mentioned  in  Part  I.,  not  very  much  has  been  learned 
regarding  the  conscious  after-images  since  Fechner's  time. 
The  mixing  of  colors  by  means  of  discs  and  the  summation 
effects  of  intermittent  retinal  excitation,  have  largely  been  con- 
sidered during  this  time,  but  in  a  historical  account  of  the  ap- 
pearances, these  investigations  have  little  place. 

'On  the  Recurrent  Image  following  Visual  Impressions  ;  Proc.  Roy.  Soc, 
LVL,  140,  1894. 


5.   /.    FRANZ. 


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ON  AFTER-IMAGES.  6 1 

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VITA. 

The  author  was  born  May  27,  1874.  He  attended  elemen- 
tary and  secondary  schools  in  Jersey  City,  N.  J.,  Columbia  Col- 
lege, 1890-4,  Columbia  University,  1894-7,  and  the  University 
of  Leipzig,  1896.  His  graduate  work  consisted  of  courses  and 
of  seminars  in  Psychology  ( Professors  Cattell,  Wundt  and 
Meumann,  Dr.  Farrand  and  Mr.  Strong )  in  Education  (Pro- 
fessors Butler,  Meleney,  Hervey  and  Reigart),  in  Anthropology 
(Professor  Boas,  Drs.  Farrand  and  Ripley)  and  in  Philosophy 
(Professor  Butler).  He  received  the  A.B.  degree  from  Co- 
lumbia College  in  1894.  In  1895-6,  and  1896-7  he  was  Uni- 
versity Fellow  in  Psychology  in  Columbia  University,  and  was 
assistant  in  Psychology  in  1897-9. 

In  addition  to  the  foregoing  monograph  the  author  has  pub- 
lished the  following  articles  : 

The    After-image  Threshold,  Psychol.  Rev.,  II.,   130-136 

On  the  Conditions  of  Fatigue  in  Reading,  Psychol.  Rev.,  III., 
513-530(1896).      (With  H.  Griffing.) 

The  Accuracy  of  Observation  and  of  Recollection  in  School 
Children,  Psychol.  Rev.,  III.,  531-535  (1896).  (With  H.  E. 
Houston. ) 

The  International  Psychological  Congress,  Science,  N.  S. 
IV.,  640-647(1896). 

Education  at  the  Psychological  Congress,  Educational  Re- 
view, Sept.,  1896. 


Date  Due 

f> 

Franz 
On  after-. 


QP481 
P85 

1899 


